If you need to flash USB in fydeos, you only need to use the fdisk command in crosh-shell, which can be used to delete partitions and format, etc.
If you need to flash another system image, just use the dd command like this:
If you have another windows PC, you can try using a partition editing tool like DiskGenius, which has a GUI, delete all the partitions on the USB, create a new partition, and then format it.