@BogMaster
Go to [Only registered and activated users can see links. ] and post back here the answers to all the 11 questions in the initial post there. Then we can proceed with a plan. The microSD may well have been corrupted at the same time but it may just not be allocated a drive letter properly in Windows.

(TLDR: The nuvi's not irrecoverable, don't bin it!). Read behind the spoiler below if you want to know a bit about how it's done:

Spoiler:
If the device can properly access preboot like you've reported, then it likely can be fixed. Modern Garmin devices hold preboot only for a very short time waiting for a flash to start. If one is initiated, then it'll stay in preboot - but if not it quickly exits as you noted. GarminExpress cannot see devices in preboot, only those which are in Mass Storage Mode/Media Transport Protocol - i.e. MSM/MTP = device visible in Windows File Explorer and Disk Management therefore not "bricked". WebUpdater.exe can use both types of communication and Updater.exe is limited only to preboot. Curing involves flashing a modified fw (cure3) as an RGN file via Updater.exe using preboot. That will enable MSM again so allowing simple removal of any faulty file/s (in your case, likely it's the most recent detail map IMG file). Then original fw is re-flashed to restore it back to normal operation so it can be again updated successfully.