Why do Apple Store insist on decrypting FileVault to run diagnostics? Much to my horror, my MacBook screen cracked the other day and had to be replaced.  I took the machine to an Apple Store and a Genius checked it in for repair.

The Genius said that they would, perfectly understandably, have to run diagnostics software after the hardware was replaced in order to satisfy themselves that everything was installed correctly.

Less understandably, he insisted that this diagnostics software (which is simply testing hardware functionality) must run from a user account in my installation of OSX—rather than, say, the recovery partition, or a wholly different installation of OSX (eg one they keep on an external drive from which they can boot if they so choose).

Fortunately, I had already enabled the FileVault 2 guest account (the one that boots to its own limited environment) and the Genius said this would suffice.  Sadly, now the repair has completed, I have received a call complaining that it is insufficient.  They are also refusing to return the machine to me without the diagnostics having been run, even though I have volunteered to take full responsibility for the consequences.

Since I have a backup of the whole computer, I suggested that they just wipe it and reinstall OSX, in order that they can then do whatever they want: I'll just restore once I get it home.  They insist that they're unable to do this without me first unlocking the drive (which obviously is not true—not least because they would solve this for customers who genuinely have lost both their passwords and their recovery keys, but also because very simple instructions are given here).

Anyway, I'm not giving out my password (which is also my key to many online resources) over the phone to a complete stranger, even if they do work for Apple.  I would be willing to go to the store to tap it in for them, but it's out of my way and the diagnostics apparently take up to 5 hours to run—so I'll end up having to make a second journey to pick up the machine once they're complete.

In all, they are making this so inexcusably painful—and frankly I'm more inclined to make a point: go there, buy an external HDD, install OSX onto it and boot that before returning it for a full refund!  (If they insist on booting from my internal drive, I could just copy the encrypted partition to the external drive, then reinstall on the internal, then copy back once all is done).

Is there any way that I can persuade them to stop being so stupid, and save us all a ton of wasted time and effort?
In my experience they don't insist anything except you ensure you have a backup of the data should service erase it.

If they don't have your password, they can boot to another OS to test. If the issue is your software, they won't be able to advise - but even knowing you have OK hardware will help you then finish the troubleshooting.

Perhaps your frustration with the repair is coloring your recollection? It would take them 15 minutes to wipe your drive and test things and if you aren't in a position to fulfill that portion of the repair agreement that's causing the delay and both sides regret entering into the agreement that wasn't "by the book". Keep in mind if they have to take hours to work around your not having a backup - other customers are delayed in their repair and maybe that along with the high volume of repairs that happen at the end of the school year.

Perhaps the time quote was incorporating the backlog and not that your machine itself was taking that long on the test bench.