Here’s the list of symptoms for those who want to go through them:
- Mac will not boot. It will halt at grey screen.
- If the external drives are connected, keyboard will not respond to the input.
- If the drives were turned off, the login screen shows up as usual.
- BUT, if the drives are connected once again, it will not allow a user to enter passwords.
- Had drives in question are connected via USB, and at least one of them is larger than 2.2TB.
I’m obviously not the only one who has that problem. Unfortunately, due to incredibly-stable nature of Mac, most of users are being told to do SMC reset, and PRAM reset. Here’s some threads (1, 2, 3) I’ve read so far. Here’s my own thread on MacRumors as well. Not only it’s obnoxious to google them, (Google wants to redirect me “from” external HDD) most of the answers are clearly not useful and most of the threads end in dread with hopeless absences. They simply stop responding to answers after certain period of times.
Here’s the info I’ve found. Although it primarily applies to Windows PC due to its legacy system using MBR (Master Boot Record), there was a barrier in HDD space that can be defined only up to 2.2TB, hence why we use GPT these days. For whatever reason, when my iMac (late 2012, 27 inch) boots up, if it picks up a hard drive that is larger than 2.2TB connected via USB, it will halt all operations. I cannot access a thing, but a grey screen. It has to remain off until I see the wallpaper.
Funny thing is, none of the drives I personally have are selectable through Startup Disk option, meaning there is no reason to inspect the other drives the startup. I can’t begin to imagine what could be causing this. But for the time being, do disconnect them before restarting your machine
update 12/13/15: info on the limit of external HDD space