I just replaced my mac mini 2012 internal hard drive with a faster SSD. I also wanted a clean install so did an install from internet. I hooked the old hard drive up to a USB adapter and when I plug it into the mac mini I get an error message that the drive is unreadable. If I try to boot from the drive (hold option while booting) it shows that there is a windows boot available (presumably from the bootcamp partition) but it apparently can't see the OS X partition.
It may be that the drive was configured with file vault but I have configured the new OS X install with my apple ID and attaching it to the same hardware so I would think it should be able to read it.