The drive datasheet will tell you it is "six axis compatible". It can be rotated to compass points, right side up, upside down and so on. You can rest the drive on its butt-end, like in a Dell or an HP. (Home built computers do not usually abuse the orientation.)
Forty five degree angles are not recommended. While it "still works while sitting on bed covers", the spec doesn't say that.
However, you may notice when the drive has 20,000 hours on it, and you're doing data recovery, that one orientation works better than another.
The only issue I'm aware of with external enclosures, was the power supplies.
There were a couple years, where the quality of 12V @ 2A supplies was just terrible. When the adapters blew, they were also ruining the drive connected at the time. That means the wall power adapter designs, lacked the most basic of protections.
This has been fixed. And the issue is no longer evident. You don't have to worry today (if buying new stock).
As for the listed "size limit", that is a limit established by testing. The computing industry has a poor track record on "theoretical size limit determination", so if a theoretical limit was listed, it would most likely be wrong. If it says "tested with 4TB", that means they tried it with at least *one* 4TB drive :-) They don't have a big budget for test.
I've just had a #2 drive suddenly develop a sticky head. When doing the first bootup of the day, when the disk was spinning up it didn't simply click quietly (as most of my collection of sick hard drives do), but instead gave six alarmingly loud clacks, before stopping and trying to spin up again. Fortunately, if left for a few minutes (presumably till it had warmed up a bit) it sorted itself out, a spun up to speed.
There was one scary moment when I set about cloning it at another hard drive. Despite leaving it for several minutes to spin up, it didn't seem inclined to do so. However, eventually it did, and the cloning went OK.
After putting it aside for a week, just out of interest I decided to try it again - and this time it spun up first time, and has continued to do so. This is what got me thinking about the effects that orientation might have (although this doesn't seem to affect the present spin-ups). X-GSmart shows it 100% error-free (despite many years of service), and for all intents and purposes it looks (and sounds) like a perfectly usable disk.
Today, there is a plastic thing off to the side of the platters.
"Three platters, six heads, six landing assists for the tab handle near the head"
formatting link
And the heads have a thing on the end (a "handle"), and as the heads move towards the ramp, the handle rides up the ramp.
This allows the heads to park off the platter.
By using regeneration (run motor as generator), the rotational motion of the platter provides energy to power the arm/voice coil and move the arm to the ramp, even on a power failure. The heads hardly ever get left sitting on the platter (as that would spell doom).
In the old days, the arm would move to a "landing zone" on the platter. The landing zone didn't have data in it. The spindle would then drop to zero RPM.
The problem with the old way, was the heads would "stick" (stiction) to the platter. And no amount of torque modulation would release them. Users would give the drive a "rap" with something, to release the heads.
Next, they tried "patterning" the landing zone, so the heads never sat completely flat on the surface. But that did not improve the situation enough to be permanently adopted.
Having ramps off to the side of the platter, has made all the difference.
While the heads are sitting on the ramp, the spindle has to be brought up to speed. If the spindle is jammed (bearing failure), the motor driver applies various electrical signals to try to shake the spindle loose. To the human ear, these are "musical tones" deedle-dee type of thing, and that noise tells you the spindle is having trouble starting. A clicking sound on the other hand, may be related to voice coil and arm, or even the arm has been left on the platter surface. But that's not supposed to happen, and tells you that there's some other kind of failure there.
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