removable_storage_test tracebacks on a read-only USB device and exits prematurely

discovered a bit of unhanded exception in removable_storage_test today...

system has an internal USB device that is read-only that contains and embedded hypervisor.

removable_storage_test encounters that and when it can't write to it it just dumps a traceback and exits, rather than handling the exemption and moving on to other USB devices as it is supposed to.

I've got a patched version of the script ready and will upload it shortly that I have tested on failing systems and determined that it resolves the issue.