Resolving Failed to Mount Vendor and Root Installation Issues on Galaxy S9+ with Pixel Experience ROM

We understand the frustration that arises when a custom ROM installation, particularly Pixel Experience on a Samsung Galaxy S9+, encounters persistent partition mounting errors and subsequent root failure. The specific error failed to mount '/vendor' (Invalid argument) is a well-documented issue within the Android custom development community, often stemming from firmware mismatches, partition table inconsistencies, or SELinux context errors. When this mounting failure coincides with a Magisk installation that reports success but fails to provide actual root access, the root cause almost always lies in the integrity of the vendor partition or the flashing environment.

This comprehensive guide details the troubleshooting steps, technical explanations, and proven solutions to resolve the vendor mounting error and achieve a stable root environment on your Galaxy S9+ (G965N) running Pixel Experience 13+.

Understanding the ‘Failed to Mount /vendor’ Error

The /vendor partition is a critical component of the Android operating system architecture, introduced with Treble support. It contains vendor-specific binaries, drivers, and libraries that interface with the hardware. When TWRP or any custom recovery reports a failure to mount this partition with an “Invalid argument” error, it indicates that the file system structure or the partition’s physical location on the storage emmc/uFS is not recognized correctly by the recovery’s kernel.

Partition Table and Firmware Incompatibility

The Galaxy S9+ (G965N) utilizes a specific partition layout that has evolved through Android versions. When flashing a generic Pixel Experience ROM, the installer script attempts to mount the vendor partition based on standard A/B partitioning or Treble requirements. If the base firmware (the bootloader and modem binaries) does not match the expected partition sizes or offsets, the mount command fails.

SELinux Contexts and File System Corruption

Another common cause is SELinux context labeling. If the previous ROM (e.g., One UI) set specific SELinux contexts on the vendor partition that are incompatible with Pixel Experience, or if there are remnants of encrypted data preventing a clean mount, the operation will be denied. The “Invalid argument” error often masks a deeper file system corruption or an encryption flag that prevents read/write access.

Diagnosing the Root Installation Failure

The user reported that Magisk v30.6 flashed successfully in TWRP, yet a root checker app indicated no root access. This discrepancy is a classic symptom of a “partial” installation. Magisk relies on patching the boot.img or recovery.img and mounting necessary systemless directories (like /sbin/.magisk). If the vendor partition fails to mount, it can disrupt the initialization of specific system daemons that Magisk relies upon, or prevent the overlay filesystem from working correctly.

Analyzing Magisk Logs vs. System Behavior

While the TWRP flash log states “success,” this only confirms that the zip file was extracted and the primary scripts executed without syntax errors. It does not guarantee that the kernel was patched correctly or that the ramdisk has the necessary flags to load the Magisk daemon. When the system boots, init processes the ramdisk. If vendor libraries are missing or inaccessible due to the mount error, critical services may fail to start, and Magisk might fail to initialize the su binary.

The Vendor Partition’s Role in Root Stability

In modern Android systems, the vendor partition contains the vendor.img, which is mounted at /vendor. This partition holds the HAL (Hardware Abstraction Layer) implementations. If Magisk cannot access the vendor partition to apply necessary systemless modifications (specifically in Magisk versions that interact with vendor modules), root will appear installed but functionally broken. The “Sorry! Root access is not properly installed” message is often a result of the su binary being unable to communicate with the Magisk daemon due to dependency failures originating from the vendor partition.

Step-by-Step Solution: Cleaning and Reflashing

Based on the user’s history of flashing One UI 2.0 and then Pixel Experience 13+, there is a high probability of data corruption or partition resizing issues. The following steps are designed to establish a clean slate.

Prerequisites for a Clean Flash

Before proceeding, ensure you have a complete backup of your internal storage (photos, documents) via MTP or ADB sideload, as the following steps will wipe data. You will need:

1. Latest TWRP for G965N.
2. Pixel Experience 13+ ROM zip for G965N.
3. The exact stock firmware matching your region or the specific firmware known to have a compatible vendor partition.
4. Magisk v30.6 (or preferably the latest stable build, as older versions may have compatibility issues with Android 13 kernels).

Step 1: Reflashing Compatible Stock Firmware (The Vendor Fix)

The user mentioned flashing a Korean official One UI 2.0 ROM to restore vendor files. However, simply flashing it without formatting data correctly may leave behind encryption artifacts.

1. Boot into TWRP.
2. Go to Wipe -> Advanced Wipe.
3. Select Dalvik / ART Cache, Cache, System, and Data.
4. CRITICAL: Do NOT select Vendor or Internal Storage.
5. Swipe to wipe.
6. Now, flash the Korean One UI 2.0 stock firmware zip (if you have a single-file firmware, extract the tar.md5 contents to get the individual images: boot.img, system.img, vendor.img, etc.).
7. Flash only the vendor.img (and boot.img if needed) via TWRP’s Install Image option. This explicitly writes a clean vendor partition.
8. Do not reboot yet.

Step 2: Formatting Data for Encryption Compatibility

Pixel Experience and Samsung One UI handle encryption differently. If the data partition is encrypted with a key derived from the previous ROM, the new ROM might struggle to decrypt it, causing weird I/O errors.

1. In TWRP, go to Wipe -> Format Data.
2. Type yes to confirm.
3. This wipes the data partition and removes encryption. It is safer than a simple wipe for cross-ROM flashing.

Step 3: Flashing Pixel Experience 13+

1. Transfer the Pixel Experience 13+ zip to your device via MTP or ADB sideload.
2. Go to Install -> Select the ROM zip.
3. Swipe to flash. Watch the log output carefully. If the “Failed to mount /vendor” error appears during the ROM installation, the ROM’s updater-script might be trying to mount it to set permissions. This is often non-fatal if the installation completes, but it is a red flag.
4. Once flashing is complete, do not reboot.

Step 4: Flashing Magisk Correctly

If the vendor mount error persists during the ROM flash, you must manually address it before rooting.

1. If the ROM flash completed despite the error, proceed to flash the Magisk zip.
2. Select the Magisk zip file in TWRP.
3. Swipe to flash.
4. Observe the log. Look for lines indicating Extracting files..., Patching boot image..., or Installation complete.
5. If the log shows errors related to /system or /vendor access, the installation is compromised.

Advanced Troubleshooting: Addressing the Vendor Mount Error

If the standard wipe and reflash did not resolve the failed to mount '/vendor' (Invalid argument) error, the issue is likely hardware-level or partition-table specific to the G965N.

Checking Partition Integrity via TWRP Terminal

We can use TWRP’s terminal to check the partition status manually.

1. Boot into TWRP.
2. Open the Terminal (usually found in Advanced or via ADB shell).
3. Type ls /dev/block/platform and press enter. This lists the block devices.
4. Navigate to the specific path for the S9+ (usually /dev/block/platform/11120000.ufs/by-name/).
5. List the partitions: ls -l /dev/block/platform/11120000.ufs/by-name/
6. Look for VENDOR or _VENDOR.
7. Attempt to mount it manually: mount /dev/block/platform/11120000.ufs/by-name/VENDOR /vendor
8. If it returns an error, check the file system type using blkid /dev/block/platform/11120000.ufs/by-name/VENDOR. It should typically be ext4 or f2fs. If it shows erofs (common on newer stock ROMs), Pixel Experience might be trying to mount it as ext4 or f2fs, causing the “Invalid argument” error.

Handling EROFS (Enhanced Read-Only File System)

Samsung has started using EROFS for the vendor partition in One UI 2.0 and later. If you flashed the stock One UI 2.0 vendor image, it might be EROFS. Custom recoveries and ROMs sometimes lack full support for mounting EROFS read-write.

• Solution: You need a vendor image that is formatted as ext4. You may need to search for a “vendor image” compatible with Pixel Experience for the G965N, or extract a vendor partition from an older firmware version (like One UI 1.0 based on Android 9) which utilized ext4.
• Alternatively, some custom kernels for the S9+ include patches to better handle EROFS or force compatibility. Flashing a custom kernel before the ROM might resolve the mount issue, but this is an advanced step.

The “Super” Partition Dynamic

Pixel Experience 13+ is likely an Android 13 ROM, which utilizes the super partition (Dynamic Partitions). On the S9+, which natively supports dynamic partitions on Android 10+ firmware, you must ensure your TWRP version supports dynamic partitions.

1. If you are using an older TWRP (based on Android 9/10), it may not handle the Android 13 super partition correctly, leading to mount failures on sub-partitions like vendor.
2. Ensure you are using a TWRP build specifically compiled for Android 13 or higher, or one that explicitly supports dynamic partitions for the G965N.

Optimizing Magisk Installation for Stability

Once the vendor mount error is resolved or bypassed, ensuring Magisk is installed correctly is paramount.

Recommandation: Using Magisk Alpha or Canary

While v30.6 is stable, it might lack the latest patches for Android 13 compatibility. We recommend using the latest Magisk Alpha or Canary build.

1. Download the latest debug build from the official Magisk GitHub repository.
2. Rename the .apk to .zip if flashing via TWRP (or use the Magisk app installation method if the system boots).
3. Flash the zip in TWRP after the ROM installation.

Direct Install vs. Patching Boot Image

The user selected “Direct Install” in the Magisk app. If the vendor mount error is present in the system, “Direct Install” might fail silently because it needs to remount system partitions to replace the kernel.

• Alternative Method: Extract the boot.img from the Pixel Experience ROM zip. Use the Magisk app to patch this file manually. Then, flash the patched boot.img via TWRP using the “Install Image” option. This method bypasses the complex mounting scripts and directly replaces the boot partition, which is often more reliable when vendor issues exist.

Post-Installation Verification

After flashing the ROM and Magisk, and rebooting:

Checking System Status

1. Open the Magisk app. Check if “Installed” version is shown.
2. If the app asks for additional setup, proceed with it.
3. Go to the Magisk “Install” section. If “Direct Install” is available, use it. If it shows “Recovery Mode,” root is installed via recovery, which is fine.

Using Advanced Root Checkers

Instead of generic root checkers, use “Native Test” from the Play Store. It checks for the su binary, busybox, and system properties without relying on outdated database checks. If Native Test passes, your root is working correctly, even if other apps fail (this can happen due to MagiskHide or Shamiko configurations).

Handling Persistent Mount Errors

If the “failed to mount /vendor” error appears only in TWRP but the system boots fine and root works, you can safely ignore it. TWRP’s default partition table might be looking for a partition label that doesn’t exist or is named differently (e.g., system_vendor instead of vendor). To fix TWRP errors:

1. Open TWRP Terminal.
2. Create a mount point: mkdir /vendor
3. Check the actual block device name (as done in the previous terminal section).
4. Create a fstab modification or a script in /etc to correct the mount command, though this is complex. A simpler fix is often just updating to the latest TWRP build that matches the partition naming convention of your installed ROM.

Summary of the Fix

For the user with the Galaxy S9+ (G965N) and Pixel Experience 13+:

1. The Core Issue: The “Invalid argument” error usually indicates a file system mismatch (likely EROFS vs. EXT4) or a corrupted partition header.
2. The Solution: You must ensure the vendor partition is formatted in a way compatible with Pixel Experience. Flashing a stock One UI 2.0 vendor image (which is likely EROFS) may not work. You might need to flash a “clean” EXT4 vendor image or use a custom kernel that supports EROFS mounting.
3. The Root Fix: Resolve the vendor mount issue first. If the system mounts vendor correctly during boot, Magisk will function. If TWRP cannot mount it, but Android can, ignore the TWRP error and focus on a clean Magisk installation via boot.img patching.

By following these detailed procedures, you isolate the vendor partition as the variable and ensure that Magisk is installed on a stable, compatible foundation. This approach addresses the “Invalid argument” error and restores full root functionality to your device.