Well, this has been very annoying for me and I am hoping to find some answers. It seems that my PSP .cso and my PSX eboots do not like to work together. Here is my story of many many many different combinations and experimentations throughout the day.
I am running 3.40 OE-A firmware, with no other modifications/programs. My library consists of 4 PSP files, and 3 PSX files, and two emulators (SNES9x and HomersGBC)
I recently bought a 4gb Memory Stick, and it works fine. I have been able to put many emulators and all that jazz on it. I was quite successful in putting PSX Eboots on it as well.
With all those in place, I put in a .cso PSP file (Let's call this PSP1). It did not show up when I tried to play it under Games. So I formatted the memory stick, and put in the PSP1 file first. The PSP1 worked.
Then I added a PSX Eboot (Let's call this PXA). Both games worked.
Next, I tried to add another .cso PSP file (Let's call this PSP2). This particular game did not show up at all under Games, yet it took up the appropriate amount of memory. It shows up when I browse the files on my computer, just not on the PSP. PSP1 and PXA still worked.
So then, I formatted my memory stick again. I transferred PSP1 and PSP2 and both those worked. Then I attempted to add PXA. It shows up, but the file says Corrupted Data. PSP1 and PSP2 work though.
So now, I format again. I transfer two PSP1 and PSP2 at once. Both those work. Then I add a third .cso PSP file (PSP3). That file doesn't show up again.
I format again, this time, I transfer PSP3 first (I've noticed that this particular file doesn't like to be last from other experimentations). PSP3 works. I transfer in PSP1. Both work. I transfer in PSP2. It works. I'm on a roll! I attempt to transfer the 4th .cso PSP file (PSP4). It transfers abnormally fast for a 500 mb file. Well, it didn't transfer in, took the memory, but no game.
So then, I delete PSP4 and transfer PXA. The other 3 PSP files are there, but PXA says Corrupted Data.
I format the Memory stick. I transfer all 4 PSP games at once. They all go in. I go into Games, and go into Memory Stick. The PSP freezes, gives me a black screen, and shuts off.
And that is where my story goes for now. Further testing is in progress. If anyone has any information, it would be greatly appreciated. It seems that there can not be an equal equilibrium between PSP games and PSX games on a single memory stick...