Removable hard drive SATA circuitry

[NoSp]

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

I'm asking about the circuit because I want to place two SATA hard
drives in a 19" rack cabinet along with an internal power supply, a fan
(with a temperature controller) and a SATA to Firewire/USB bridge board
(http://www.span.com/catalog/product_info.php?cPath=1308&products_id=7656).

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.

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.
So that's why I've been looking into removable hard drive cabinets,
because surely they can't be harmful to the drives, and they do indeed
cut/supply power to a drive. If I could just replace the physical
removal of the drive bay with a switch it would seem perfect for my task.

 

[NoSp]

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?

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.

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?

 

[Baron]

NoSp Inscribed thus:

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 agree !

I suppose "unmounting" does the flushing of caches which you talk
about,

It does !

so after unmounting the drive properly, would it harm the drive
to cut its +5 and +12V lines via a relay?

If you "unmount" the drives first and then switch them off it would be
no different to replacing the on/off switch with a relay

 

[NoSp]

NoSp Inscribed thus:


If you "unmount" the drives first and then switch them off it would be
no different to replacing the on/off switch with a relay

That's great!
However, with another external hard drive enclosure of mine (factory
built, not a DIY project like the one I'm discussing here), the rear
on/off switch is attached to the bridge board itself (ATA to
Firewire/USB). A DIN connector is also attached to that board which an
external power supply "brick" supplies +5V and +12V to the bridge board.
But I suppose it's no more complicated than the switch cutting power
from the DIN connector. I'll take a closer look.

In another thread someone suggested I use a latching relay for my
project, because that'll enable me to use a momentary switch (to
"trigger" the relay), and at the same time "remember" if I had that
particular drive switched on or off last time I used it.
(the reason I want to use momentary switches is because the majority of
switches that look "high tech" and aren't too big don't come with
locking mechanisms, but are momentary.
Ultimately, a "power button" logoed type button (as used with Apple
Macs) for each drive would be ideal for this sort of thing:
http://www.wap.org/journal/powermacg5/pm5frontlg.jpg

Do you know where those (or similar types) can be bought?

Back to the relay though; from another thread I'm under the impression
that unlike a simple "off-locking on", and "on-locking off" positioning,
the relay "wiggles" a bit back and forth from its on and off states,
leading to the hard drive getting a series of on/off/on/off/on/off...
actions -this surely can't be healthy for a hard drive (or any
electronic device for that matter)!
Is there any truth to this, or does a latching relay simply and firmly
just switch from one position to the other, and locks?

How many Amps should the relay be able to handle at the +5V and +12V
lines for each SATA hard drive?