XQD card reader no longer working after upgrade from Mojave to Big Sur

Over the past couple of years I have imported thousands Nikon NEF (RAW) files from my camera into my iMac using an XQD card reader. I have imported both into Photos as well as into Capture Pro 20.

Since my last import some months ago, I did a long-avoided upgrade from Mojave to Big Sur. In any case, just the other day I tried to do a fresh import and discovered that the XQD reader does not seem to be working. That is, the camera volume is not showing up either in Photos or in Capture Pro to do the import.

However, I can import files with Photos by using a USB connection from the camera into the iMac. It's pretty slow but it does the job.

It's possible, of course, that the explanation here is that the XQD reader has gone bad in the interim. Doesn't seem likely, but who's to say?

I could simply throw in the towel and have B&H send me one of their new card readers that also read CF Express cards. But that's an expensive way to troubleshoot!

Any suggestions on what might really be going on here?

Thanks.