I have a 2012 imac and I was running High Sierra on it. I was going to sell it, so I erased it using the recovery system. Never had a problem before, but this time when I went in to reinstall, it says that the disk is locked. I can't find a way to unlock the disk, all the options in disk utility are greyed out except "first aid" and "partition" - but it won't let me partition either. I am completely at a loss. I already did the Internet Recovery and my computer gets stuck on the apple logo after the internet recovery screen finishes - it was there for 3 hours last night and never moved on from there. Any help appreciated.