Ok, I think I've broken it - having not touched it (or any RAID since 2014!) anyway... here is the situation:
Motherboard: Asus H81M-K
2 x 2TB Disks
CMOS Battery died / replaced.
Booted but came up with PXE-E61media test failure.
I removed, re-seated the cables.
Same message.
This just means no boot device or network boot available.
No RAID options in the BIOS where I would have hoped to have seen them (Under SATA config?) or anywhere else.
Upgraded BIOS to latest version. - Nothing changed.
Reset BIOS to defaults - Nothing changed.
The RAID option should be in the BIOS if you go into advanced mode, then go to advanced -> SATA config -> SATA mode selection. Then you can choose RAID.
If you kept one drive unplugged to keep the data on it safe while you fiddled about with the other, you should plug that one in alone and boot from it after the changes in the BIOS. This should work as from what you have said you have never actually gone into the BIOS and reset its SATA configuration back to RAID. Any working drives that will still have the data but you won't be able to boot from it until you do this.
When other drives are plugged in, you may need to adjust the boot order in order to boot from it and rebuild the mirror using the Intel RST application. Make sure you know what drive is what and what is on it so you don't copy the broken one onto the saved one.
It is possible that it lost its RAID settings due to the dead CMOS battery. I have also seen a Windows update reset this setting on more than one PC in the last year. It is also possible that someone decided to update the Intel RST RAID drivers. I have seen old machines that will not boot with newer drivers, but those drivers are required by recent (last year) Windows updates and will leave the system in an unbootable state and you have to revert to a ACHI setup.
Desperation (and stupidity / ignorance) = I have a full backup of the server and they need it back up for files. So I attempted to reinstate as 'normal' (i.e. no RAID) and rebuild one of the disks seeing as I'm pretty sure I've lost it anyway with my messing about and now it won't install Windows as it was a GPT configuration... so reset the disk to normal MBR... and
Well... those of you who have had the patience to read this far will be shaking your heads and saying to yourselves 'why oh why oh why'.
But if anyone has any advice on how to even get Windows booting so I can get them a machine back of sorts - even without RAID - that would be great. Meanwhile I'll continue to read forums and dig deep into the recesses of my technical knowledge and see what I can come up with (not a good idea but not sure what else to do!).
Thanks in advance!
If you want to go down that road, make a
Windows boot USB from Microsoft, boot off that, format all partitions as part of the installation. You will lose all your data from that drive.