Do hard drive enclosures fail/is it the HDD or enclosure

I'm having a whole host of problems with an external hard drive that was working just fine a couple of hours ago. I've had this problem before once, and that was about 3 months ago, here's what I documented:

So a couple of hours ago I turned off all my computers and shut off
the power to all my devices in my room, then went and turned the power
off at the main switch so I could change an outlet.

A couple hours later, after I've already slowly turned everything back
on, I go to my xbox to try and watch a movie and it can't seem to list
any of the movies I've got. So I go to my desktop to find that my
external hard drive isn't there.. even though it's on and connected.
It's also stationary and hidden behind something so there's not a
whole lot of tampering/physical wear to that external.

I plug it into my laptop to try and see what's going on. It starts
making this endless loud screeching noise. None of that clicking
that's usually associated with hd damage. It's not listed in my
computers, and it shows up in Disk Management as "uninitialized"
asking me to choose between two different partition types. After
carefully disconnecting it and connecting it back, it asks me to
format it, which I cancel.

I start googling about my issue, starting to accept the situation,
torn as hell and helpless and just about ready to toss the thing.

Suddenly the screeching stops, after almost 45 minutes of it going,
and Disk Management lists the drive as "Online" and "Healthy".
Explorer pops up with all my files! I'm still being really careful
with it and weary and treating it as though it's in fragile shape.

I've downloaded some S.M.A.R.T. software to read the values and
everything is listed as "OK" . No reallocated sectors, no read errors,
no seek errors. I also ran a quick self-test, which completed without
error. Everything seems fine.

It looks to be a perfectly healthy external hard drive.

So what the hell was that about? Was it doing some sort of maintenance
or self-test? How am I supposed to tell the difference? I would've
undoubtedly killed the drive for sure if had it gone on a bit longer.

I've got the same problem now, with one exception: it doesn't magically reappear after the screeching stops. Occasionally I manage to get some S.M.A.R.T. diagnostics information, which basically reads everything as fine. The only problem is that my HD isn't initializing (so I can't access anything in it). I'm able to successfully run a quick smart test but not an extended one (I've only tried it once but got conflicting indications as to whether it was actually making any progress or not (was stuck on Random read test).

So, final question (if all else fails): Could the hard drive enclosure be failing rather than the HDD? Is this a likely possibility at all? How would I know?

[edit]

So after lots and lots of attempts at trying to get my data, trying many combinations of different computers, turning off the host before disconnecting HD, disconnecting HD and then disconnecting the power, and vice versa etc etc I finally got my drive to "initialize". Again.. just as last time, my files are all still here, I've verified that they're all complete using MD5 checksums (ExactFile). The drive is still alive and healthy according to disk management and S.M.A.R.T. Just to document again, I did many things before my drive came alive again (nothing significant) but here are the last few: I carefully removed the hard drive out of the enclosure (which was a bit dusty), and connected a spare laptop HD to the enclosure to verify that it wasn't the enclosure (I didn't want to buy a new enclosure for nothing), everything loaded up normally. It wasn't the enclosure.

I figured since I'd pretty much lost my data, I could screw around a bit anyway without worrying about recovery, and I decided to connect the external directly to my laptop (SATA) and boot up live Ubuntu. Ubuntu couldn't find my HD, and I don't think it even turned on, maybe because it required more power than my laptop provided. I just disconnected it and put back my laptop's hard drive and tried using the enclosure again with my external. Same problem with a slight difference: No hard drive size, just uninitialized. The external wasn't screeching during this time, just the silent idle sound. So I disconnected it (just USB), and the blue light on the enclosure turned off, but then the HD started to screech again. A few seconds after it started screeching, I connect the USB back in. After a few seconds of weird HD sounds (maybe 30 seconds), explorer pops up with all my files. To verify that they were back permanently, I "safely remove"d the external and disconnected the USB, waited til it turned off and then reconnected it, the files popped up again. I still hear lots of screeching, and it's constant. But it doesn't sound like the disk is being scratched, so I think it might have something to do with the spindle. Either way I'm going to back up my data asap.