Linux – How to make a USB-docked hard drive not “disconnect” from the connected system when idle

root@ubuntu:/home/ubuntu# sg_readcap --16 /dev/sdd
READ CAPACITY (16) not supported

This means when your USB docking translate the capacity from the drive's ATA IDENTIFY DEVICE data (seen in hdparm -I / smartctl -i), it can at most report a size up to 32-bit (i.e. 0xffffffff, 4294967295) in terms of number of logical sectors. This is an inherit limitation of SCSI READ CAPACITY (10):

Logical Sector Size | Maximum capacity supported (TiB / TB)
         512        |              ~2.0 /  ~2.2
        4096        |             ~16.0 / ~17.6

Since your drive is an AF 512e drive that has totally 5860533168 / 0x15d50a3b0 512-byte logical sectors, which requires 33 bits to represent, only a SATA/USB bridge that supports SCSI READ CAPACITY (16) can handle it properly. When the size is truncated to 32-bit, it turns from:

101011101010100001010001110110000 (5860533168)

to

 01011101010100001010001110110000 (1565565872)

The Linux kernel, or probably all OSes, will basically never issue ATA IDENTIFY DEVICE command "directly" (i.e. encapsulated in a SCSI ATA PASS-THROUGH command) to USB drives, but SCSI READ CAPACITY commands (which you issued manually with sg_readcap), to get the capacity of them.

Only when the drives are actually SATA drive connected with a SATA/USB bridge, the command will be handled by the SCSI-ATA Translation Layer implemented in the bridge, which will then issue ATA IDENTIFY DEVICE command to the SATA drive to get the information it needs to form the response data for the READ CAPACITY command.

But utilities like hdparm and smartctl are (almost) exclusively for ATA drives, so they pretty much do everything with ATA PASS-THROUGH. (Also because they are userspace utilities, it is expected you, the user, will only use them on the appropriate type of devices.) That's why you end up getting different capacity in different places.

You should get the tool "Process Monitor" from Sysinternals. With a few filters set, you can filter out anything unrelated and just look for the accesses on your hard disk which you don't want to spin up.

I'm currently having the same problem, with the Windows 10 Creator's Update from this month, it's even gotten worse. Maybe once an hour or every two hours, my other HDDs are spun up, for no apparent reason. HDD timeout is set to 20 minutes, but that doesn't mean much if they randomly spin up again later!

I'll have to do the same and look with Process Monitor what actually causes this. It's most likely something within Windows like svchost, which will be hard to prevent.