I know this is going backwards with technology, but I'lll post it since a lot of users still use the Garmin 60CS/x, 76CS/x, and older models of the Etrex GPSs and it might be helpful for someone.
I load quite a lot of my custom maps and other's custom maps on to friends GPSs and recently I had a problem getting my North Cumberland Trails map to load properly on a Etrex Legend HCx and also on my 60CSx. The custom .typ file was being recognized in Mapsource, Basecamp, and on my Oregon 450 but no typ file was being seen on the older GPSes where all maps have to be loaded at once to the gmapsupp.img file and can't be renamed. The map was there on the GPS just not colored per the typ file.
I was loading 17 different Mapsets to a 2Gb card. Total maps was around 848 and about 1.73Mbs, so well within the limits of the GPS. The maps included Metroguide, US Inland Lakes, US Topo, RWS Caribbean v2.5 plus 13 more custom maps and all maps appeared properly except the one... North Cumberland Trails. To make a long story short and skip all of the changes I made to the properties of the North Cumberalnd Trails map, 13 custom .typ files was the maximum number these older GPSes would recognize whether it was due to a number limit or size/memory capacity for "drawing" being reached I never fully determined, since I got the .typ file to be recognized by reducing the number of custom .typ files/mapsets being used. It was very repeatable (100%) with the Mapsets I was loading.
The official Garmin maps did not affect the .typ file count number. The RWS Caribbean v2.5 mapset utilizes 3 typ files - for different map levels. All other custom maps each had 1 custom .typ file.
If anyone else has experienced the same problem using a lot of custom .typ files on older GPSes and can add more information to this, I'd be curious to see what you determined.