Dear ku4hx - First off, I wanted to thank you for the potential solutions and help you threw my way yesterday. Reading continued threads in the Apple Community and searching other resources, I found that I had a USB hub plugged into a port on my 2018 iMac 27". Apparently, when I upgraded to Monterey, this USB hub somehow blocked all my external HDDs and my XQD outboard and SD card (built-in) reader as well. I unplugged the USB hub, turned on my outboard HDDs, and put cards in the readers. Everything came back beautifully. Then I unmounted everything, plugged in the USB hub, and re-mounted everything without a hitch. It was my electric kool-aid acid test. There is something in Monterey that does not like for a USB hub to be connected when one migrates from Big Sur to Monterey. I don't know what it is, but hopefully, someone else who has had this problem will get to this and try it. It was terrifying, as a photographer, to have my work locked up on HDDs that wouldn't mount. So there you have it. Try it and see if it works for you if you're having a similar problem. And thanks again ku4hx. You're awesome to come to the rescue with your suggestions.
