I have been using an external HDD for backup my main pool. I attach the drive to a dock that is then recognized by TrueNAS when I power it up.
The dock only work with SATA2 speed. At SATA3 speeds I always get CRC errors and what not…
I have added a line in the /boot/loader.conf
hint.ahcich.5.sata_rev=2
which downgrades the interface to SATA2 every time the HDD is attached. I have to add this line everytime I upgrade.
My questions is, how can I do this in TrueNAS SCALE and Linux?
Have you checked your BIOS to see if it has the ability to limit the connection to SATA2? That would be an easy way to solve the problem. SATA2 speeds are plenty fast for a typical NAS.
Haven’t thought of that option, thank you, I’ll find out.
There is no option in BIOS to change the SATA speed.
Found this option in kernel parameters:
libata.force= [LIBATA] Force configurations. The format is a comma-
separated list of "[ID:]VAL" where ID is PORT[.DEVICE].
PORT and DEVICE are decimal numbers matching port, link
or device. Basically, it matches the ATA ID string
printed on console by libata. If the whole ID part is
omitted, the last PORT and DEVICE values are used. If
ID hasn't been specified yet, the configuration applies
to all ports, links and devices.
Should work when i find a right place for it
On Core there’s a section for “Tunables” that you can add such settings. I assume it will be in Scale somewhere too, mine is under the System category:
Can someone on SCALE confirm this?
Docs seem to say so:
Edit: Though it doesn’t seem to make any distinction of where the sysctl will apply. I assume it will apply when needed? 
Some experimentation required…
I think sysctl could work with libata parameter…