Is this a custom case? Possible the custom case isn't compatible.
After router is at the ready, connect USB drive, power off the router for 10 sec then power back ON, does the router see the drive? Does the LED on the drive flash any while the router is booting?
Review the router logs for USB entries after the router gets to ready. If you don't see any entries, then I presume there is a compatibility issue here. Not all custom cases meet USB standards.