There is mgba and Virtual Console injection (with New Super Ultimate Injector for 3DS). Both use the SD inside the console. Two things about this:
You might (??) be able to use GBArunner with TWiLight Menu++ to run ROMs from the micro-SD inside the R4.
- GBA ROMs are fairly small (32MB maximum); there's a good chance you can still put some of them onto your card. (If using injection, pay attention to the 300 titles limit of the Home Menu, don't inject a ton of ROMs)
- Bigger SDs are no problem if you format them to FAT32, 32KB cluster size. Cards as large as 256GB have been reported to work (and bigger cards probably also would). Do not use 64KB clusters on a 64GB card. This leads to graphical glitches in GBA games. On larger cards 64KB clusters are okay (some people reported a slight speed improvement with 64KB clusters on very large cards).
Thanks, will look it over in the AM.