Is the map on the SD card recognised by BaseCamp? If it is, you may have Windows on your computer set to hide file extensions for known file types. If that is the case, it will hide the .img extension of the map file. If you are not aware of that, and add the file extension yourself, then in actual fact the full name of the file has become gmapsupp.img.img (the computer only showing gmapsupp.img). Verify your computer settings as follows:

Displaying the File Extension in Windows 7
- Click Start (bottom left corner)
- Type "folder options" (without the quotes) in the search box.
- Select Folder Options from the results list.
- A dialog box with the title Folder Options will appear.
- Select the View tab.
- Uncheck (untick) the box for "Hide extensions for known file types".
- Click the "OK" button at the bottom of the dialog box.

After having verified you are using the correct settings, check the file name again and make sure it is gmapsupp.img and not gmapsupp.img.img.

If using another Windows version, instructions will be similar but may differ in detail. Use Google if necessary to find the right instructions.


Edit:
I see loly was thinking along the same lines as me while I was writing my post.