I have recently been having trouble with very slow scan speeds.
In the past, scans would start at around 100 MB/s and end at ~250 MB/s which is consistent with the inner and outer track speeds for HDD's.
However, the 8 new hard drives I installed this weekend have had scan speeds consistently in the ~40 MB/s range with no improvement over 24 hours.
As an anecdote, 4 of the hard drives were WD Elements 10 TB externals. When I initially scanned them in their enclosures, they all averaged ~160 MB/s from start to finish.
After shucking them and placing them in my Supermicro JBOD, I had to rescan them (the enclosure has a different device name) and the scan speeds were the ~40 MB/s previously mentioned.
2 of the hard drives are HGST HC510 10 TB drives. When I tested scans in the JBOD, they averaged ~40 MB/s. However, when I tested 1 of them with a USB to SATA adapter, the scan speed was about ~120 MB/s
*Note, scan speed seems to vary with number of disks scanning and whether a USB drive is scanning as well. With all 8 disks scanning, the speed is slower, ~25 MB/s, at 4 drives, it is ~40 MB/s.
If I have one of the HC510 drives scanning from the USB adapter at ~120 MB/s, the other 3 drives only scan at ~20 MB/s
If the HC510 drive from the USB adapter is NOT scanning, the other 3 drives scan at ~45 MB/s
I have attached images showing the disparity (no speed data on the USB adapter so I measured the scanned amount over 1 minute from the block map)
