I had the same happen when upgraded one of my servers and doing this fixed it:   Backup your flash drive then 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 -restore files from flash backup -execute make_bootable as admin