Source ps2nfo.com:
I found a way to make DevHook work. DevHook can't load after u press start only on Sony MS PRO Duo High Speed. But it works with other cards. So u need another card (working). Most of us gained default 32 mb ms pro duo with psp. Use it.
U need : 2 cards HighSpeed (not working) and working 32, DevHook 0.41b and Game (or demo) UMD.
1. Copy same devhook to each card.
2. Insert 32 card to ur PSP.
3. Load devhook. Options :UMD Select: Umd Disc, Firmware: 2.50+2.00 (set it), Boot Select: Reboot XMBUMD Version: 2.XX -> 2.00, and CPU Clock: 333 MHz / 166 MHz.
4. PSP reloads and autoruns your UMD game. Quit it.
5. Insert ur big card.
6. Select Game -> UMD (DON'T RUN IT!)
7. Change ISO with Home+ L/R trigger. Picture hasn't changed.
8. Run UMD, and when u see white background and PSP logo, open ur UMD-Drive.
9. U see error message. Close UMD-Drive.
10. Select Game -> UMD and u can see picture of game u want to run. Do it .
Je n'est pas testé car c'est plutot long comme méthode! Esperons qu'un patch va bientot sortir...