Ok, have been poking around the BIOS for the best part of an hour & still no Joy getting windows to boot again (although at least nothing I change seems stop OSX booting which is great)
My Motherboard (MSI z270 SLI Plus) doesn't seem to have any obvious 'CSM on/off' setting, but does have 'Legacy+UEFI' boot mode select. This is always been on. There is also a dedicated 'Windows 8.1/10" switch...this is usually off, but when its 'on' I can only use UEFI boot mode..I have tried every combination of these & still no joy from Windows. Sometime though I get a tiny square windows logo stuck at the centre of the screen....sometimes the flashing cursor....
..ive also tried booting via the Clover USB with these settings & it makes no difference
..the mystery to me is why Windows10 has been booting ok these last few months...any more thoughts ? & if I *do* have to just wipe it & reinstall Win10, I don't remember the install USB stick ever giving me a choice of installing a 'EFI Partition'...it just asked if I wanted to install or not...are there deeper options?