Maybe you need to merge at5.xml files from 2 vector maps manually, and put result in root of sd? It helps in some cases.

I've never made a vector map, so not sure if those will work on a Raster map or not. Thanks for the tip though. Might try it. In the mean time, I'm going to try 3 or 4 smaller Raster maps again, load 'em, and just see. It worked once before, so not sure why it won't now, unless the files were still in the HDS somehow.

Vector is the best way for large areas.

If you have the source shape files, you could create e. g. all US american waters chart (fresh water and open sea together) for HDS (valved under 10 GB).

Some folks from this forum created a few big charts with >> 1.000 km² area, or >> 100 single lakes. The sources was e. g. datas from the government (lucky chance).

For raster is this much to big. The problem is the SD-card size and the processing time. For a few 100 km² as high quality raster the computer can need days ... weeks (or crashed), and the file size can exceed 64/128 GB.

For about 150.000 km² vector the IMC needed < 1/2 h (< 60 MB).