Question GPU not showing up under devices on non-sff Optiplex 990

[RJ-45-Dude]

(I don't know if this has been covered exactly yet, as others' situations don't seem to match mine. Also, didn't know if I should post this under GPU or motherboards, so I just put this under GPU)
So, recently, I purchased an Optiplex 990 (not sff). It came with a 500w PSU, so I thought it would be able to handle my R7 260X. I put it in, and nothing - no output, it doesn't show in the BIOS, it doesn't show under device manager. I update the bios, I switch to a 600W PSU, and nothing. So, I make sure that it's not a gpu problem, so I plug the 260x into another PC, and it works. So, to see if the PCIE slot is bad, I plug another GPU into the 990, an HD 6570, and it shows up in BIOS and device manager. So, I plug in the 260x back in, and it shows up an BIOS at boot, but disappears when I get into Windows, and then when I restarted the PC, it never showed up in BIOS or Windows again. I even tried Linux to see if it would recognize the card, but no.

I'm at a loss. The GPU is fine, the slot is fine, the PSU is fine, I don't understand what's happening.
Any help is appreciated!

 

[tennis2]

The R7-260X and onward are UEFI GPUs and will not be supported.

The Nvidia GeForce 10 series were the last ones to be non-UEFI. A GTX1060 (or similar 10xx) should/would work.

Or there might be a workaround for UEFI GPUs. Google around.

 

[RJ-45-Dude]

Wait, so if I'm understanding correctly, if I were to set the BIOS to UEFI (it's currently in legacy), the 260X could work?

 

[tennis2]

It's free to try. I don't recall if/what switching from Legacy to UEFI may cause issue with, but certainly nothing irreversible.

 

[RJ-45-Dude]

OK, I tried switching over to UEFI, and it didn't work. However, found my Intel ME settings - would disabling the SOL, IDER, or KVM help at all (sorry if this is a stupid question, I don't know much about ME settings)?