Re: Removable hard drive SATA circuitry

Joel Koltner wrote:
"NoSp" <NoSp@xxxxxxxxxxxxxxx> wrote in message news:4857d717$1@xxxxxxxxxxxxxxxxxxxx
What kind of circuitry is used for a removable SATA hard drive system such as this?: http://www.addonics.com/products/mobile_rack/aenrhdsa35.asp

Not much, I expect. They mention it only works with certain controller chipsets and operating systems, so I imagine the way it works is that you hit the little "please let me remove you now!" button, it generates an interrupt and the chipset driver flushse its caches and then tells the drive to spin down. It probably doesn't even cut power from the drive itself -- once the motor is stopped and the heads are parked, physically disconnecting it from power is as good as cutting it via a MOSFET or relay.
It sounds a bit more complicated than my suggestion of just cutting power via a relay though.
Why is it important to flush the drive's cache?


All of this would be straight-forward if it weren't for the fact that I want to power on/off the two drives *individually* -because I seldom need to use both of them at once. Think of the setup as two separate hard drives which just happen to be physically placed in the same box.

Could you just tell the drives to spin down instead? When not running, hard drives use very little power.
Yes, spinning *down* is no problem, but preventing the drives from spinning *up* when power is applied is more difficult.
Power isn't the issue here -it's that I want to prevent tear and wear, and also keep noise levels down.
Like I said, it's seldom that I need to use both drives at once.

My first thought was simply to use a dual-pole relay coupled with a switch for each drive. The relay would cut/supply power to the +12V and +5V lines, but I've been told that such a crude setup could be harmful to the drives.

It's possible but unlikely, IMO -- most hard drives are designed these days to be able to use the energy from spin down (when the power is cut) to park their heads, so as long as you weren't in the middle of writing any data, you'll still be OK. It is "crude," certainly, but I don't believe harmful. However, I'd suggest you ask your question in one of the more "computer"-related forums.
I thought about asking in a more computer related group/forum, but they seldom know much about electronics at a component level there.
My system will consist of an Apple Mac. The way an external drive like this works is that in order to "disconnect" it from the computer you eject or "unmount" it by dragging the drive's desktop icon to the trash/eject icon. That'll spin down the drive's motor and its desktop icon will disappear.
In order to "reconnect" that drive I need to physically turn off its power switch, then turn it on again, which will spin up the drive and automatically "mount" it on the Mac desktop. The drive's icon appears on the desktop when "mounted".

If I simply turn off the drive's physical power switch an error message pops up on the Mac, telling me that I've wrongfully disconnected a drive, which would cause damage to its data, so I should never cut power before "unmounting" it first.

I suppose "unmounting" does the flushing of caches which you talk about, so after unmounting the drive properly, would it harm the drive to cut its +5 and +12V lines via a relay?
.

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