Running captive tests via SAT may not work because smartctl does not pass a long command timeout to the SCSI pass-through ioctl. And even then it would require that the SAT layer in the USB bridge firmware supports such long (several hours) timeouts.

Non-captive tests may be interrupted by a standby command sent to the drive after some time of I/O inactivity. Many USB bridges do this.

Try to run some script which accesses the drive during the test. This may work or not:

while true; do
   dd if=/dev/sdX iflag=direct count=1 of=/dev/null
   sleep 60
done