I have two Optiplex 780 machines running Windows 7 Pro. Both do the same thing. Since I have an eSATA port, I bought an external HDD for backups. I switched the bios from legacy (SATA drive) to Raid/ACHI, rebooted and then changed the bios to include external eSATA. Rebooted and I get a repair screen or start normally. If I try to start normally it reboots to the same screen. If I do the repair it hangs up forever until I shut it off.