Psp 352 M33 Upgrade To 660 Portable link -

Upgrading a PlayStation Portable (PSP) from the ancient firmware to a modern 6.60 custom firmware (CFW)

Follow the on-screen prompts. Do not turn off the PSP while it writes.

Step 4: Finalizing

Fix: Ensure your ISOs are in /ISO/ (capital letters) on the root. Filenames must be short and contain no special characters (spaces, brackets, etc.). Use names like KH_BBS.ISO.

And That's It!