Hmm. Sounds like there may be a problem with the sata ports. If you move the one working drive around on all the ports does it get detected on each port? If no, some may be disabled or faulty. If the one working drive gets detected on each port then you may have some faulty drives, or faulty cables. You could also try using the windows installation media to see if that might see the drives, and use it to format the drives to gpt.
 
https://docs.microsoft.com/en-us/windows-hardware/manufacture/desktop/windows-setup-installing-using-the-mbr-or-gpt-partition-style

https://www.thewindowsclub.com/convert-mbr-to-gpt-disk
 
Maybe one of the following tools can detect them, and help you convert them to GPT.

If you are using UEFI bios but the HD's were originally legacy formatted, they will not show up. As long as one of the drives is showing up you should be able to use EaseUS Partition Master to to detect and reformat the other drives.

Try setting up the bios for legacy drives even though you won't be able to boot up. Might want to save your current profile first. Then save and re enter the bios and see if the drives show up in ahci mode. At least you will know if they are dead that way.

I think I've bricked them. The SSD's were taken out from my old ASUS Hero Maximus VII board, and placed in this new machine which has a different power supply. I didn't even know you could destroy SSD's in that way. I also may at one stage used my old SATA cables from the ASUS motherboard, again, not realizing that that could be a problem. Let's just say I've had a rude education on the frailty of SSD's.
 
I tried connecting them to (x2 SanDisk Ultra II 960Gb) to another PC and they aren't seen on that one either. I also tried the suggestion of disconnecting the SATA cable connecting it to the port on the motherboard, and leaving the SATA power connection plugged in to potentially reset the drive - again to no avail.
 
Open to other suggestions, but at this point, the reset failed, the BIOS doesn't see them in any machines, EaseUS, DiskPart, and AOMEI Partition Assistant don't see anything - I don't think there is much else I can do except accept the fact that they are now bricked and dead!

JimBeamBlack
I had a similar experience with my Dark board, only I only had 2 drives that could not be seen by BIOS or windows, it turned out to be the wifi, if I had it disabled 2 of my SATA drives didn't show up, I had to enable it (even though I'm not using it at this time) in BIOS and after a reboot all drives show up.
 
Maybe something similar is happening here?
 
 

 
Yes - this is mentioned in the Manual  - [I looked after seeing Your Post]
(excerpt page 144)
 
"Likewise, if there is a device that shares bandwidth with your SSD or HDD (Page 27), make sure that the desired device is enabled in BIOS, or all other troubleshooting that comes after this section is moot."

Thanks for that tip, I tried disabling the M2 Key E Wifi slot and it didnt make a difference. Double and triple checked the cables, no luck. The only thing that bothers me here is that I have 2 older drives that are SATA but not SSD's, they also aren't seen by the BIOS. The only drive I can get it to see is an old SATA non-SSD drive thats already pre-formatted to GPT format. Not sure if thats a coincidence or not. Also, and forgive my lack of knowledge here, but how do I find out if any devices share bandwidth? Thats about the only thing I think I have left to check.