Hello. I just tried to erase the Macintosh HD partition on my Late 2015 5K iMac and it came up with an error and said something like 'Cannot erase...etc.'. The Macintosh HD partition has now disappeared and rebooting the system doesn't change that, the system boots straight into OSX Utilities instead of OSX and if I try to reinstall OSX the only partition/drive that shows up is Recovery HD and nothing else. In Disk Utility it has the Fusion Drive listed under Internal but nothing else underneath that. Apple disk image Media is listed under Disk Images and OS X Base System under Apple disk image Media. Both First Aid and Partition options don't seem to be of any use. It is a 2TB Fusion Drive.