Unfortunately USB 3.0 and Wireless in the 2.4 GHz and even Bluetooth are interfering in the same frequency spectrum, which often causes problems. See PC Mag Article about USB3.0 and 2.4 GHz wireless issues.
The problems caused by these interferences do not affect the drive activity, but can seriously impact your 2.4 Wi-Fi, if there is an USB3.0 storage device attached to your OpenWrt Wi-Fi device:
These problems will most likely also appear, while the drive is idle.
Workarounds may vary for your situation, you may try: