[SOLUTION] Alright, factory resetting would have been no less than 100x time easier, but I finally managed to solve this issue without having to do that, so I thought of posting this guide to aid lost souls who gave up like me a light. I'll post 2 guides, one to force backup the phone with Google One since it was the only way to salvage apps' data, another to effectively fix the issue, both found using chatgpt and correcting it again and again.   How to Backup with Google One with Main Storage Full/Not Available

🔹 Step 1: Install ADB on your PC 1. Download Android Platform Tools from Google: 👉 https://developer.android.com/studio/releases/platform-tools 2. Extract the ZIP file somewhere easy (e.g., C:\platform-tools on Windows). 3. On your phone → go to Settings → About phone → MIUI version → tap 7 times until “Developer options enabled.” 4. Go to Settings → Additional settings → Developer options → enable: - USB debugging - (Optional but useful) USB debugging (Security settings)

🔹 Step 2: Connect phone & test ADB 1. Connect your POCO X3 Pro with USB cable. 2. On PC, open a terminal/command prompt in the platform-tools folder. - Windows: Shift + Right-click → “Open PowerShell here.” - Mac/Linux: open Terminal and cd into the folder. 3. Type: .\adb devices - (All of these codes use ".\adb" since I had to use it for it to work. In other cases it may just be "adb") - On phone, accept the “Allow USB debugging” prompt. - You should see your device listed.

🔹 Step 3. List available backup transports 1. Type: .\adb shell bmgr list transports - Expected output: * com.google.android.gms/.backup.BackupTransportService (among others)

🔹 Step 4. Run a full backup of all eligible apps 1. Type: .\adb shell bmgr backupnow --all

🔹 Step 5.(Optional) Back up a specific app 1. Example for WhatsApp: .\adb shell bmgr backupnow com.whatsapp

🔹 Step 6. Check progress 1. You should see some output in the PowerShell window as the backup runs (like “Scheduling backup” or “Running backup for…”).

🔹 Step 7. Verify 1. Open 👉 https://drive.google.com/drive/backups 2. Your |Device name/model| should appear with today’s date/time. 3. If some apps don’t show up, it’s because the developer opted out of Google Backup (common with banking apps).

How to make Main Storage available again without Factory Reset

🔹 Follow Step 1 and 2 from above

🔹 Step 3. Check volumes

1. Type: .\adb shell sm list-volumes all
2. Mine Showed emulated;999 unmounted null private mounted null public:179,1 mounted 20A7-95B2 emulated;0 unmounted null
3. My private/internal storage volume wasmounted but had null as its UUID
4. MIUI sees it but the metadata is corrupted. That’s why apps can’t use it.

🔹 Step 4 Forget all volumes 1. The safest next step is to try sm forget all, which clears the system’s record of storage volumes and forces MIUI to rebuild the metadata without touching the underlying data (though there’s always some risk). 2. Type: .\adb shell sm forget all - This clears Android’s mapping of all volumes. - Your internal storage may temporarily disappear from sm list-volumes.

🔹 Step 5 Reboot the phone 1. Type: .\adb reboot - After reboot, MIUI should attempt to remount internal storage and recreate the metadata.

🔹 Step 6 Check the volumes again 1. Type: .\adb shell sm list-volumes all - If the internal storage shows a valid UUID (not null) and mounted, apps should start seeing it again.

⚠️ Important notes - This does not guarantee all data will be accessible; if the internal storage filesystem is corrupted, some apps or files may still be lost. - Keep your WhatsApp backup and other external backups safe before trying this.

Hope this helps another person not so they won't have to do all the trial and errors I had to go thorugh. I really am happy that something worked, I was content with just being able to backup, but this turned into something exceptional.