a USB flashdisk formatted by mac may be unreadable in windows.
first, you can try “disk management” in wondows, see weather you can change anything.
but if you used the USB flashdisk as disk for mac “time machines” (or others), thus, the flashdisk will be un-change-able in wondows “disk management”.
Now, you can type DISKPART in “search” or in “command prompt”
DISKPART> list disk
DISKPART> select disk 1
then refresh the “disk management” and create a new partition. done.