Ensure your PSP battery is at 75% or higher. During the flashing process, a power loss will brick your device. Play it safe: keep the AC adapter plugged in.
Direct upgrade from 3.52 → 6.60 is impossible. The NAND flash layout and IPL (Initial Program Loader) signatures changed in 5.00 and 6.00 OFW. A two-stage process is required. psp 352 m33 upgrade to 660
Now that the system core is updated, we patch it to run homebrew and backups again. Ensure your PSP battery is at 75% or higher