d850 .NEF unsupported all of a sudden

[Atoms_SanFrancisco]

I have found myself in a bizare error/unrecognized predicament. Camera files Nikon Raw .nef from my d850 saved to SD cards (I have now run through three different cards as trials) aren't recognized to import on an 2015 iMAC.  I run two "software" identical computers, a 2015 intel iMac with Retina, and a 2019 MacBook Air . Both on Big Sur, Both most up to date versions of Lightroom Classic 10.1.1. The MacBook recognizes flawlessly, the iMac just doesn't recognize. Other cards filled from a d500 storing NEF and from a Sony storing .AWR are normally recognized on the iMac.  Today I shot 5 shots in .nef, 5 shots in .tiff on the D850 with a BRAND NEW UNFORMATTED CARD, then put it in the D500 and shot a few in .nef, NONE WERE RECOGNIZED. Anyone with ideas? I am somewhat computer illiterate to even remotely know what difference between intel dual core i5 MacBook Air, and a quad core i5 iMac that would cause this issue? 

[dj_paige]

The difference between what is happening on your MacBook and what is happening on your iMac almost certainly comes down to a hardware malfunction somewhere. I suspect the problem is the card reader (but it could be other things).

[Atoms_SanFrancisco]

This is a great idea to problem solve, would that still hold true if I can download other file types from other cards? I have put several cards into the iMac and been able to get nef files off, as well as AWR raw files, it seems to only be an issue with the images/files from the D850. Can a reader have file specific troubles? Should I get a peripheral SD card read and try it?