Thanks can anyone recommend a file browser app to use to load roms?
well I think the XBox comes with one preinstalled... but I'm not sure. there are very basic ones and then there is the paid app called "My Files Explorer" (you can test it for a day or so to see if it works for you) which is to my knowledge the only file browser app that has access to almost every file on the XBox, other file browsers are usually limited to the "My Files" part of the internal drive... so Documents, Music and Video folders basically.
I think you can even use the VLC Player app as a file browser... haven't really messed with that but I think there are tools in there to copy and paste folders etc.
the issue really is, that you need access to the install folder of Retroarch for some emualtors! I use the FTP app you need to be whitelisted for to acces those, and the only app on the store that I know of that can access these is the "My Files Explorer" app which costs around 3 bucks I think.
now why do you need access to these folders? well, some Retroarch Cores will not like it if bios files aren't located inside the "system" folder inside the Retroarch install path. I had issues getting some PS1 cores to run, and Amiga had issues as well. not sure how many cores will have issues that way, but I have to say I only tested these using a USB drive, I didn't have the bios folder on the internal drive/ inside My Files on the Xbox when I had issues... so maybe it works if you have them internal in your My Files folder, you have to test that.
but again, there is a 24h test version of that "My Files Explorer" app, so maybe get all the bios files, get that test version and copy them over in one go, and just to be save get every possible bios you need on there in one go as well. just so you don't need to buy the app.