Am I understanding right that it continued shocking you after you had unplugged it from the wall? If so, the PSU is definitely toast, probably dangerous, and I would make no further effort to debug that if I were you.
If that's what's going on, the energy for it is most likely coming from the APFC cap, which needs to store enough energy to cover the PSU's rated output (550W?) for something less than 1/120th of a second. It usually does this at something like 400V. This capacitor can kill you.
I've cracked open PSUs before (which people will rightly advise against, and nobody should do unless they're very sure they know what they're doing), but I wouldn't want anything to do with one that's doing what you describe. That cap obviously isn't directly shorted anywhere, but if power from it is following an unknown and unintended path at voltages you can feel, something is broken enough to be scary.
If I were in this situation (and am understanding the situation right), I would handle this PSU with one hand only (so if it does give you a bigger zap it won't go to your heart), set it somewhere out of the way for a few days (or however long you feel is appropriate) to try to make sure it's discharged, then throw it away. I would definitely NOT ever plug it back into the wall.
Edit for one other theory - it could theoretically be coming in through an ethernet cable or something. I'm not familiar enough with that to say much more on it.
Either way, if you were getting repeatedly zapped when messing with hard drives, the problem has probably toasted a lot of other components by now.