You haven't stated your unit model, but this is universal for an SD/microSD card: x:/Garmin/gmapsupp.img. Just rename it. Most modern units can also have a folder named Map on sd or unit where you can have multiple maps with 'anyname'.img. Please read: [Only registered and activated users can see links. ] particularly the links under 'Storing maps in devices'.

PS: No need to quote entire long posts, just the relevant bit.