Old thread, but for those still experiencing this error, I want to add that I got a "CheckForMultiGameAndRegion() failed: 1" trying to boot Gamecube titles from USBLoaderGX after replacing my flash drive with one of a higher capacity/read speed.
My scenario was: I had just bought a new, bigger USB flash drive, formatted to FAT32, copied over my files/folders from the previous & working 256gb to the new 512gb (same model, SanDisk USB 3.1), and even though the files were identical, GC roms wouldn't boot via USBLoaderGX on the new flash drive.
They boot from the Nintendont app just fine, however. This meant the USB isn't necessarily incompatible or corrupted, but that USBLoaderGX isn't reading from it correctly. After searching and skimming threads like this, I almost gave up and returned my new flash drive for a new one thinking it was incompatible.
But, after some messing around, I found the fix was simply changing the setting "Remove Disk Read Speed Limit" within USBLoaderGX from "OFF" to "ON". It seems that my USB read speed is faster than expected for loading GC ISO's. Hope this helps someone.