I have a semi-tricky config: booting Mojave MacOS off internal M.2 SSD number 1 ... booting Win10 off internal PCIe SSD number 2...
All this was working fine yesterday. Could boot into Clover and then select either the Mojave or the Win10 disk, and Bingo, up comes the selected OS. Reboot and yes, back into Clover and select disk and away we go. I thought I'd really got the hang of this dual booting thing.
I disabled auto-updates in Win10, had heard that they could stomp the EFI partition.
I left Win10 up for about 24 hours, installing some S/W, running benchmarks and malware detectors etc. Then after a bit of relaxing gameplay I decided to switch back to Mojave.
Which I have done before, yesterday. Please note I did not touch Win10 core or EFI partition or change anything OS-configgy.
Imagine my cussing when my mobo would not boot to clover, or even directly into Win10. It got in an endless boot-into-BIOS thing, where no matter how I set my EFI boot priority list in the BIOS menu, when I Save and Reset I end up right back in BIOS every time! Finally had to plug in a USB stick to get Mojave back (BIOS did acknowledge and boot the EFI on the stick. thank goodness).
So I'm here to testify that this problem is not specific to older boards. I have no idea what causes it, but it's bloody frustrating when it happens and so far I have not found any sure cure. I wonder if I should update my BIOS (scary!) -- haven't wanted to touch it for fear of breakage, but looks like it's broken pretty bad so maybe not much to lose? Board's only been in service about 3 weeks.
And yes, before y'all ask: Secure Boot is OFF, Fastboot is OFF, CSM is OFF. And there is a perfectly good EFI partition on the main boot disk (I just checked it).
So anyway... this is really annoying and I wonder if anyone has a clue 'cos I am fresh out.