New
#41
Windows Boot Manager should be set as first Device to boot in BIOS. It is what boots an EFI install, not the hard drive listed.
Does the problem happen with the MyBook attached? Try without it and all other peripherals attached.
If you're saying that the startup hang at boot screen does not happen with the disk in the drive, then that normally means the boot files on the hard drive are corrupt. Try running a few Startup Repairs.
So we now have the old install to Legacy and fresh reinstall to UEFI causing the same problem? Let's look at the hard drive. Run a full Disk Check followed by the maker's HD Diagnostic . Do you have another HD to try in the PC? An SSD is the greatest upgrade one can have now.
BIOS P11-A3 appears to be the latest, but says its for Win8. I wonder if it brings UEFI up to latest. I don't think I would do it unless you can find the present BIOS version to restore if the latest causes problems.