Which scheduler are you using on the USB device? Did you try deadline or BFQ?
There is a long standing (CFQ-related) kernel issue causing system lockups in high I/O situations:
There is a long standing (CFQ-related) kernel issue causing system lockups in high I/O situations:
With blk-mq + nvme and later lightnvm that whole IO-scheduler mess will hopefully go away over the course of the next couple years (at least for personal computers).