Tested with just one map, with and without GarminDevice.xml and gmaptz.img also... No results.

Mine is 4.10b, latest, double checked. SD is a Samsung SDXC 64 GB MB-SGCGB. [Only registered and activated users can see links. ].

While in the GPS, the card can be read/written by PC over USB normally. Diagnostics > Screen 'Data Card Test Page' gives Checksum: Fail and RW Test gives no results.

I'll try with a slower, smaller SD.