All I can see is that all six drives are on the -same- SCSI bus. This should eliminate ahci(4) or scsibus(4) as root causes. I'd look for pin configurations on the drive -- see section 3.2 of the Barracuda 7200.11 manual, where it shows limiting speed with a jumper block.
(Disclaimer: I don't have one of these -- I just looked it up online.)
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