Try re-creating the flash drive manually:   If it's not a new Unraid install backup flash drive first.   Open a command prompt window as administrator, then type, in this order:   -diskpart -list disk -select disk x (x=your flash drive) -clean -create partition primary -format fs=fat32 label=UNRAID quick -active -assign -exit -close cmd window -download Unraid release zip and extract to flash (or restore from backup) -execute make_boot