When the drive does not handle any input/output activity during the test, it may go to standby, which raises the
Interrupted (host reset) condition. Try to read from the disk at suitable intervals:
while true; do dd if=/dev/disk1 of=/dev/null count=1; sleep 60; done
/dev/disk1 with the appropriate device; reads one sector from that device every 60 seconds until you hit
This helped in my environment: OS X 10.6.8, WD Elements USB-connected drive, SAT-SMART-driver 0.8.
A captive test should theoretically keep the drive online. Yet the hardware command send by
smartctl may time out before the test completes, causing the kernel to reset the link and ending up in the same situation as above (bug #303).
See this thread on the smartmontools-support mailing list for further details. I acknowledge Christian Franke for the insight given here.