What does “-501 0D” mean when trying to run the Apple Hardware Test? I have a late 2012 iMac running El Capitan which has become quite sluggish. I’ve tried a few software solutions, but I think it’s something wrong with the disk. (A lot of stuff is fine; anything that touches the disk starts crawling.)
I tried to run the Apple Hardware Test using Apple’s instructions.
I booted my Mac while holding down the D key. I got the “Starting Internet Recovery” screen, then I was asked to select and authenticate with a Wi-Fi network. I did that, and it went back to the “Starting Internet Recovery” screen. Shortly afterward, it got stuck on an error:


apple.com/support
–501 0D


I’ve tried Googling this code, but I can’t find anything on what it means – mostly just some Apple Discussion Forums with other people asking about it.
Anybody here know the answer?
Brief follow-up note: it turns out this Mac did have hardware problems.

I took it to the Apple Store a few days after writing the original post, and their more thorough diagnostics discovered a problem with the disk (specifically, the hard drive portion of the Fusion&nbsp;Drive). I got the hard drive replaced under warranty, and after a year I haven‘t seen the symptoms again – I assume this was just an artefact of a failing disk.