Ok, I wrote a clean 3.1 to the SD card, expanded the filesystem and copied

SCPH1001.BIN to the folder /home/pi/RetroPie/BIOS

The filesize of the file I used is 524,288 bytes and has an MD5 of

that worked fine with the PSX games I tried, it worked fine upper or lower case.
It couldnt find the bios file if I renamed it to a random filename.

What is the MD5 of your BIOS? Does it match any here?