X360 - Jasper revision: modded
PS3 fat model w/250gb upgrade
Wii: soft modded w/320gb backup
PSP 2004: 5.50 gen-C prom
PSPgo : 6.37ofw w/27 converted isos
3DS w/SSF4, waiting on the edge of my seat for ocarina 3D and Res. Evil: Mercenaries 3D
I made the same mistake when I first tried an ISO. The most probable reason why those ISOs/CSOs don't work is because they require a certain firmware to be installed on your PSP.
I highly recommend custom firmware 3.40 OE-A by Dark Alex. Sad news is though, Dark Alex has thrown in the towel, as it were, so that will be his last custom firmware. You can't get it on his site anymore (as far as I know) so you'll have to Google yourself another source. Good luck.
First, hold R when powering on the system to go to recovery mode.
Then go to configuration and change the 'Use no UMD' to 'enabled'.
OR.. alternately, leave it at 'disabled', but keep a GAME disc in the drive.
If you're playing psx games, make sure the homebrew kernel is set to 3.40 in advanced settings, and for emulators, put them in game150 on the memstick.
Lastly, if ever playing ratchet and clank: size matters - set the cpu speed to 266mhz.
Try not to take all this in at once, as it might seem overwhelming. Just refer to it as needed.
X360 - Jasper revision: modded
PS3 fat model w/250gb upgrade
Wii: soft modded w/320gb backup
PSP 2004: 5.50 gen-C prom
PSPgo : 6.37ofw w/27 converted isos
3DS w/SSF4, waiting on the edge of my seat for ocarina 3D and Res. Evil: Mercenaries 3D
X360 - Jasper revision: modded
PS3 fat model w/250gb upgrade
Wii: soft modded w/320gb backup
PSP 2004: 5.50 gen-C prom
PSPgo : 6.37ofw w/27 converted isos
3DS w/SSF4, waiting on the edge of my seat for ocarina 3D and Res. Evil: Mercenaries 3D
Oh, and yes you can always go back to 1.5 from 3.40 OE-A and in fact any custom fw (as far as I know, it's always worked for me). Although, if it all works fine for you, at the end of the day you shouldn't really have to resort to 1.5 again.
Having said this, if you want the absolute most up-to-date on firmware emulation, skip 3.40 OE-A and download the custom fw 3.52 M33 (with Update #2 if needs be) from the M33 team. I can't vouch for the stability of this firmware as I've never tried it myself and don't plan to unless a really awesome game comes out requiring 3.5x. So it's up to you, it depends entirely upon what it is you want from your PSP. Just follow all the instructions you're given with whatever cfw you might want and your PSP will be dandy!
