White banding on iMac while logging in. What does it mean? I have a late 2012 iMac which is behaving sluggishly.

I rebooted, and when I try to log in, it hangs for a long time (it’s hard to make out, but it’s rainbow pinwheeling), and then I get a weird whitewash on the screen.

It goes all white, and then the login screen gradually fills in from the bottom. Here’s a picture of it; click through for an animated GIF showing the whole thing:



I suspect this Mac has a hardware problem, but trying to diagnose it with Apple Hardware Test has been unsuccessful.

I haven’t been able to find anything similar on Google for this behaviour, although perhaps I’m not using the right search terms.

Does anybody recognise this behaviour, and know what problem it might indicate?
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.