I'm building a small server, and I was planning to add a smaller disk to perform backups. The problem, here, is that ideally the smaller disk should be turned on just when performing the backups and then shut down when the backup is completed.
Since the motherboard supports SATA hotplug, I was planning to just put some high power MOS (in fact they are more complex switches, but let us consider it just a PMOS) in series with both the 12V and the 5V rails going to that HDD. This way it should turn off.
Now, is there any problem with this approach? I mean, the data cable will be always attached, and I'm just removing the power. Can this cause problems?
The disk is a 3.5" HDD (7200rpm)
Best Answer
The question is effectively: "can SATA controller tell the difference beween SATA port connected powered down drive and SATA port not connected to anything?". I think the answer is no, so you should be fine.
(1) SATA cable only has 2 differential pairs, so there is no CPD or any other out-of-band information that can be used to tell if drive is plugged in.
(2) One arbitrary SATA PHY datasheet suggests that receiver only cares about presence of differential signals, and not about common mode or transmitter resistance: http://electronix.ru/forum/index.php?act=attach&type=post&id=607 says:
(3) The same datasheet has no mention of receiver detection on transmitter part.
I am going to guess that other PHY's behave the same way, and thus you can safely power drive off while leaving it connected.
This said, are you sure this is what you really want? There may be easier alternative solutions:
hdparm
to put the drives into sleep mode. No hardware required.