Your unit may have a 128MB limit for the data card. The GPSMAP2006 was not listed but the GPSMAP2006C is listed.
Was not able to post the link so here is some info from Garmin's FAQ.
03/01/2011
Older versions of Inland Lakes mapping could be preloaded on a proprietary Garmin Data Card. These Garmin Data Card versions are now discontinued and no newer versions will be made. The versions that were preloaded to the Garmin Data Card were programmed on a 256MB data card. Some devices are only compatible with a Garmin Data Card with up to 128MB of space. This means that the preprogrammed Inland Lakes cards cannot be used. These devices are:
◦GPSMAP 172/172c
◦GPSMAP 178/178c
◦GPSMAP 182/182c
◦GPSMAP 188/188c
◦GPSMAP 232
◦GPSMAP 2006c
◦GPSMAP 2010c
◦GPSMAP 2106
◦GPSMAP 2110
Mark
Was not able to post the link so here is some info from Garmin's FAQ.
03/01/2011
Older versions of Inland Lakes mapping could be preloaded on a proprietary Garmin Data Card. These Garmin Data Card versions are now discontinued and no newer versions will be made. The versions that were preloaded to the Garmin Data Card were programmed on a 256MB data card. Some devices are only compatible with a Garmin Data Card with up to 128MB of space. This means that the preprogrammed Inland Lakes cards cannot be used. These devices are:
◦GPSMAP 172/172c
◦GPSMAP 178/178c
◦GPSMAP 182/182c
◦GPSMAP 188/188c
◦GPSMAP 232
◦GPSMAP 2006c
◦GPSMAP 2010c
◦GPSMAP 2106
◦GPSMAP 2110
Mark