Be sure 420s firmware is up to date or may be it cant recognize sdhc .
Have you formatted it as fat32 ? How big is that card? Some plotters have problems in reading big size sd.
Better to use normal and cheap 2gb sd.
Your map is copied ok in a Garmin folder, Check with Gmaptool in Details log if it's properly unlocked.
If it's locked you'll see this in log:
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