Old Dell Inspiron will only boot UEFI from F12 Menu!

[Porthos]

It turned out there was a mini-SATA drive presumably setup as a paging drive that went south since the SSD replacement.

That is an optane drive.

 

[Rigo]

That is an optane drive.

No, it was Sandisk brand
Thought Optane was intel only stuff
Didn't show up anywhere, not in BIOS, nor under the Diagnostic utility that only showed the main drive that passed the test, not under Disk Mgmt in Windows or Speccy. Just no-existent except for confusing the boot loader

 

[Porthos]

Thought Optane was intel only stuff

Was this an Intel-based computer?

 

[Porthos]

HP Touchsmart 23 AIO cache drive - Google Search

 

[Markverhyden]

It turned out there was a mini-SATA drive presumably setup as a paging drive that went south since the SSD replacement. That mini-SATA was not mentioned anywhere in specs or hardware repair manual but just hunting for what could be the problem I saw it on the mobo and removed it just as another attempt.
BOOM!!
That was the culprit!
Put back the original SSD, all good!
So I'm keeping an eye open for this kind of crap from now on.

Good catch. I've seen those recently on server class motherboards. Many OEM's will mount them with their extensive diag tools, etc. Far better than using USB like some had years ago.

 

[Rigo]

Was this an Intel-based computer?

Yeah

 

[Rigo]

HP Touchsmart 23 AIO cache drive - Google Search

Good catch, somehow my search terms didn't give me the results you got when I was struggling with the beast.
That's the one indeed