Right ... beer-o'clock over there so best you don't touch it in a heightened 'Bud' state. I still think there's something on it which is causing the aberrant behaviour, eventually it may brick completely. It will be much easier to load cure fw and reformat it now while you can still boot it but it's up to you of course [read the note above regarding 7xx series, they can be a bitch to enter preboot when bricked].
Unfortunately because you got it 2nd hand you don't know the history of it. Someone may have formatted it with the Windows built-in tool, or with a Mac, or in Linix. Only ever should do Garmin internal flash format using [Only registered and activated users can see links. ], nothing else can be certain of a completely acceptable FAT format for Garmin's proprietary OS.
EDIT: BTW, your NA unit has [so called] 4 'GB' in total flash memory. By necessity some of that's used for the OS and other essential memory 'regions' but even if the visible file system is empty Windows will report ~3.65GB of free space available [it's actually GiB, i.e. Gibibytes, Win still uses binary measure so that's roughly around 3,920,000,000 bytes or ~3.92 decimal Gigabytes]. The different binary [factor 1024] and decimal [factor 1000] usages of the term 'GB' can cause confusion over what space should be available. Linux for sure nowadays [and i think Mac] reports as decimal for KB/MB/GB/TB etc.
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