Mass Storage or Region-only terms are only referencing how the device operates. Modern Garmin automotive/handheld/offroad navigators are quite different in their basic operation to that of all classes of earlier devices. Many marine devices operate on that original principle of having specific regions (like little separate partitions) in their memory, used for dedicated storage i.e. they are Region-only. For example, in region-only devices, the basemap file is in region 3, on-board primary detail map usually in region 49, voices in region 17, etc. The other type (Mass Storage) has a visible file system where all such essential basic files including map *.img files are contained in one large region, usually 48 or 83, and then other small multiple regions have particular purposes. That main region is browsable by computer, as is the media card when it's inserted too. However, 'region-only' devices treat the card as just another region, usually it's 10. That's quite certainly the case with your chartplotter seeing it doesn't even have a USB socket.

Now that you understand how your chartplotter works, you'll realise that if there's a problem with its flash memory that it's not possible to re-format it like a Mass Storage Mode (MSM) device to overcome file corruption, bad blocks etc.. So even if in fact there is only some very simple corruption within the memory there's no easy way to correct it. In your case maybe it's a hardware failure though, it might be the flash chip has a physical fault rather than a soft corruption.

The "can't unlock maps" message means that the on-board primary map/chart itself is being seen by the device but the associated *.UNL file (containing the unlock code) for the *.IMG map file, is either corrupt, unreadable or missing entirely from region 49 (or wherever it's stored in your plotter).

It simply isn't possible to dump (i.e. copy) regions to check contents and then re-upload clean files when the device lacks a way to connect directly to a PC and there's no functioning media card slot.