Instructions
Turn on your PSP and hold the "R" button. Your PSP will boot into recovery mode.
Press the arrow keys to highlight "Configuration"; then press "X". Navigate to "UMD Mode" and then press "X."
Highlight "m33 Driver -- No UMD"; then press "X." This will disable the UMD function that is interfering with your ISO file.
Press "Start" and then press "X" to exit recovery mode.