never had that problem with 3.90, just make sure the umdiso driver is set to m33 driver and you may want to try updating to a later custom firmware,; latest is 5.00 m33-6
its also possible he has a fake memory stick, i would try formatting it with the psp and then check its properties by going to pictures,movie, ect and pressing triangle to see if its magicgate supported
so cso's work fine on his but not iso, and same memstick on yours both iso and cso work... thats kinda strange but like i said i would try updating the firmware, makeing sure the right no umd iso driver is set to m33 driver and also you could try going into the recovery menu and resetting the flash0 and settings
