I had to step a 2012 MBP that was running HS with AFPS to a previous time machine backup that was also HS with AFPS (just a few days ago). Upon reboot I get the circle with the line through it. Tried it a second time. Everything seems to go fine except that the machine is not bootable when it's finished. I can run first aid on the SSD (in the 2012 MBP) either from recovery or by connecting from another (High Sierra) machine via target disk mode and it checks fine. So I decided to reinstall the OS and simply migrate the stuff in from the backup. But apparently you can't install High Sierra via Target Disk Mode! I guess I have to make a usb installer. What a drag....