The stack of drives-to-be-scrubbed disgorged a pair of SATA drives, so I plugged one of them into an internal SATA port and unleashed
dd on it:
time sudo dd if=/dev/urandom of=/dev/sdb bs=4096 count=10000 10000+0 records in 10000+0 records out 40960000 bytes (41 MB) copied, 4.19793 s, 9.8 MB/s real 0m4.208s user 0m0.004s sys 0m2.880s time sudo dd if=/dev/urandom of=/dev/sdb bs=1024 count=40000 40000+0 records in 40000+0 records out 40960000 bytes (41 MB) copied, 7.38392 s, 5.5 MB/s real 0m7.394s user 0m0.004s sys 0m3.505s time sudo dd if=/dev/urandom of=/dev/sdb bs=16384 count=2500 2500+0 records in 2500+0 records out 40960000 bytes (41 MB) copied, 4.2042 s, 9.7 MB/s real 0m4.214s user 0m0.000s sys 0m2.880s
The timing for a few (tens of) thousand blocks comes out somewhat below the long-term average, which was the same 12 to 14 MB/s produced by the USB 2.0 adapter. It just doesn’t get any better than that, mostly due to rotational delays.
In round numbers, that’s 40 to 50 GB/h, so it’s best to start scrubbing those terabyte drives early in the day…