I tried three computers, including a round trip of 60 miles to the third, using USB leads that I knew worked fine as I had updated my Montana and an XT using them. I also knew that the 3rd computer I tried had updated the owners Nav V unit last year. Alas all to no avail, regardless of which storage mode I tried the Nav V in.
Frankly I had all but given up, assuming perhaps that the unit itself had a problem that was beyond my limited knowledge. My last ditch attempt was going to be taking it to yet another computer which was running Windows 10 as opposed to 11 on the three tried so far.
For this attempt I rooted around in my "tech" box and managed to find yet another USB lead. Just before leaving home I thought I would give it one last attempt on my computer using this as yet untried lead. Yes, you've guessed it!!, it connected the Nav V to Garmin Express seamlessly and updated with no further issues.
So thanks to everyone who contributed ideas and workarounds. I'm still baffled why it wouldn't update earlier but I guess that's technology for you
.