Thanks for the help.

I’ve been running it through HDMI. I disabled the variable HDMI resolution and tried it again, first using the HDMI cable, then switching to composite. Neither one seemed to work–selecting roms still flashes to black briefly, then takes me back to the menu.

For most emulators, the result is something like this:

/home/pi/RetroPie/supplementary/runcommand/runcommand.sh: line 15: /home/pi/RetroPie/emulators/gsps/raspberrypi/gpsp: No such file or directory

For MAME it’s different:
Gauntlt1.6p NOT FOUND
ERROR: required files are missing, the game cannot be run.
RetroArch [WARN] :: [GL]: Stock GLSL shaders will be used.
RetroArch [ERROR] :: Implementation does not support save states. Cannot use rewind.

All roms and MAME files (romset 0.145, but the version doesn’t seem to make much difference)work on other computers. Even when installing an additional .chd file for MAME games, I get the same result.

I ran the setup script from the beginning to see if an updated version would change anything. Now the problem seems to affect only Sega (Megadrive) and Game Boy Advance.

I’ve found it’s a pretty complex problem. You have to have MAME files from the Mame4all romset, but you also need to have files for the same game from different romsets as well, and you can’t rename any of the files or use ES Scraper for MAME. I don’t understand why it works that way, but I’ve found if I change any one of those things, I can’t play the roms anymore.