Excellent. It took a few tries, as UMDGen is prone to shutting down on me and copying the files I drag and drop into it multiple times, but I finally got it saved and working flawlessly. Thank you so very much for your help! I'll keep my eyes and ears open to see if I can learn more about how this all works, then perhaps I can return the favor some day.
You'll need to first dump your UMD to your Memory Stick... which we don't cover in the UMDGen Forum. Check THIS thread for the info and tools used to do it, and there is also a PSP UMD Dumping Guide in the STICKY thread for UMD Dumping as well.
I suspect you are using a 'pre-patched' BOOT.BIN file. If you open the BOOT.BIN in a hex editor, search for 'fatms:' and it produces matching text then the BOOT.BIN has been patched - you will need to replace every instance of 'fatms:' to 'disc0:' if there is a folder name other than PSP_GAME after fatms: then that will need to be changed also.
Example: fatms:/ALPHA/ would need to be changed to disc0:/PSP_GAME/