Hi Adam,
As I expect, that this isn’t an issue regarding a faulty thumb drive plugged into the board, I supsect that it has to do with the fader circuit boards, which, as far as I know, are also connected to the GLD main board via USB.
There has been a forum post some time ago with pictures from the inside of the GLD.
Maybe this helps: https://community.allen-heath.com/forums/topic/inside-gld80 but I think you will have to make demands on the A&H support or on your dealer.
Best regards,
Dieter