If you are encountering this error, here is how to fix it:
Check Folder Permissions:
Clean the NAND (Last Resort):
Once you’ve resolved the issue, adopt these best practices to avoid it happening again: dolphin iosfs failed to write new fst
Before troubleshooting, it helps to understand the terminology.
So when you try to change the contents of a game ISO—for example, replacing a texture or adding a new file—Dolphin’s ISO builder or Filesystem Editor must rebuild the FST to reflect the new file locations and sizes. If this process fails, you get the “failed to write new fst” error.
Once resolved, follow these best practices to avoid seeing the error again: If you are encountering this error, here is how to fix it:
A. Run Dolphin as Administrator
Right-click the Dolphin executable and select Run as administrator. This mitigates most permission-related failures.
B. Check Free Disk Space
Ensure the drive containing your ISOs and Dolphin’s cache folder has at least double the size of the game ISO free. For a 1.35 GB GameCube ISO, keep 3 GB free.
C. Disable Read-Only on the ISO
Right-click the ISO file > Properties > Uncheck Read-only > Apply. Check Folder Permissions:
The virtual NAND might be corrupted.
Fix:
If your Dolphin folder is on a network drive, external USB, or cloud-synced folder (Dropbox, iCloud Drive), move it.
How to change the User Folder path: