i have a late 2013 15 MBP with max RAM and 750gb ssd. Running on latest Mavericks and fully backed up on time machine. I tried installing yosemite and after install reboot. i get some small type messages on the apple logo screen that appear for a few seconds and then disappear. The progress bar goes to about 1/3 complete and then I get a black screen with a circle and slash. I zap the pram. No effect. i restore to mavericks and fix permissions and run disk first aid. Try the install again. No little print now but same logo then circle with slash screen. i try preparing an usb Yosemite install. I erase the SSD. and it installs and seems to work. i use the the assistant to migrate the data from my time machine. At the end of that it asks for a reboot and comes up with..... The black screen with a. Circle and slash. i reboot in verbose and the final message is waiting for root drive. Or I think its something like that. i erase the SSD and do another usb install. But no migration. It works? Reboot.... It works. But why would a migration cause this circle with slash? Any other ideas. i have had macs since 1984 and I have NEVER had this much problem with a new system!