2 and a bit years - so out of warranty. However, I might try the old faithful "Consumer Rights 2015" - not fit for purpose, blah blah. I'll give a windows machine a go as well.
I think it's a Mac / Garmin problem.
As I say I switched to a Windows PC and it all worked immediately.
So I suspect that Garmin express is broken with a recent Mac OSX update or something like that.
Especially as the SD card can be seen in OSX and written to.
If you have a Windows PC lying around try that.
What nav are we now talking about, the loan unit or your one, your posts are a little confusing. i will presume you have the complete maps installed on your Mac, if so have you tried using map install to see if you can see your card using that. and if so install the maps via that method. link to map install below, try this and report back.
https://www8.garmin.com/support/download_details.jsp?id=3825
i dont understand how you are able to use the maps downloaded for your unit on the loan unit, i thought they were device locked. or are you using different maps. Sorry but this is all a little confusing you say Quote (The issue I am NOW getting with Unit 1 is that its telling me there's not enough space on the computer) was this the first mac or the second mac or the windows machine.
Quote (I've had a look at Garmin MapInstall, it recognises both the unit memory AND the SD card....but doesn't think there's an unlocked map in either) is this because you have a locked map selected in basecamp, or have not selected an unlocked map, you are trying to explain so many scenario's that i am struggling to follow this.
on the mac thats telling you there is not enough space to install maps onto the mac, why is this have you no room on the hdd, can you not remove some stuff. if map install can see the unit and sd card and the map you are trying to install is unlocked it will install it, but you say its locked, iff this is so you would not be able to install it on any device.
Post a screen shot of basecamp showing your maps and there locked and unlocked status, without the codes listed of course.