OC220 Won’t Recognize USB for Backup

My OC220 will not recognize a known working USB key formatted as FAT32 for a backup target. When enabling the backup schedule there is a informational context stating "Backup Schedule is available only when it's powered by a PoE device and a storage device's USB port." I meet both the PoE and USB requirements when enabling the backup schedule.

Do you have any thoughts on why this might not be working?