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One of my mac started becoming intermittently unresponsive, so much so that I couldn't even manage to get it to restart regularly. After a hard reset, the pre-boot authentication screen of filevault 2 came up as usual, but would then get stuck (I gave up waiting after 20 minutes) on the screen with the logo and spinning wheel.

Booting from an external disk, I tried unlocking and mounting the disk via Disk Utility - the unlock took about 10 minutes during which Disk Utility mostly appeared unresponsive (rotating colored wheel, red status in the activity monitor).

Eventually the disk appeared in the Finder, and I discovered that navigating into a folder can take about 1-2 minutes! Surprisingly, when I tried copying files that went extremely fast (just as normal I'd say).

How would I go about fixing this problem? (I really wish to avoid reinstalling the machine, for various reasons)

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Step 0: Clone the actual encrypted bytes of the disk somewhere safe in case it's a hardware issue. Skip only if you have good, tested (!) backups. –  Daniel Beck May 28 '12 at 16:47
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Use a tool like SMART Utility (shareware -- free to use the first few times) to see if your disk is failing. If you have any pending, reallocated, or remapped sectors, then your disk is failing and you should replace it. You have to actually look at these counters because disks will report "SMART Status: Verified" even if they're having these failures.

You can actually see these counters from Mac OS X 10.7.x Lion (or later) Disk Utility, if you select the drive itself (not a volume/partition, but the drive) in the source list and select "Get Info". But SMART Utility makes it a little more foolproof.

If you see nonzero counts in pending, remapped, or reallocated sectors, then your hard drive is failing and you should get everything off of it that you can, test your backups, and then replace the drive.

You might be able to squeeze a little more life out of the drive if you cause every sector the the drive to be written to. You can do that by erasing the entire drive with "Zero Out Data" enabled. Hard drives don't remap their own bad blocks until those blocks are written to. So if you have a bad block in some location that gets read from a lot but never written to, like some place where low-level filesystem information is stored, then you can have the symptoms you're experiencing.

If you do the "Zero Out Data" erase/format thing, your drive may become usable again, but you shouldn't trust it. You should figure it's "circling the drain" and about to have a cascade of further failures.

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