I understand that it may seem confusing to understand all the questions on what can be done and not be done with different Garmin gps models!
Some basic info valid for your 60CSx which is an excellent device, allthough with a little too small screen for MC use...
The 60 CSx can only have one file with map(s) but it is possible to have many maps in this file.
The file must be named gmapsupp.img and be in the Garmin folder of the sd-card.
The size of the file can be max 2GB (or is it possible with 4GB with latest firmware? I have a faint feeling I have read it somewhere, someone can possibly clear this?)
In your case I would install the maps you are intersted in Mapsource (or Basecamp) as a first step. You will anyhow need the maps in your PC to be able to plan your trips and transfer them to the gps.
After having installed the maps you can select the parts of the first map, then switch map in Mapsource and continue selecting the the parts of the next map.
When done with this you can transfer the map either direct to the gps or with the patched version of Mapsource to your PC:s harddrive, a file named gampsupp.img will be the result of this. If you transfer direct to the gps the old file gmapsupp.img will be overwritten!
If transferred to PC you can then copy the file to the sd-card but using the usb will take loong time if the map file is big since usb1 is slow.
In the gps you can now select the map of interest, you should be able to select all the maps that you transferred but normally select only one mat at the time. The cpu of the 60CSx is not the fastest in the world but the batteries lasts long!
Of course it is possible to have several sd-cards with maps and switch the cards in teh field but normally you would be able to get all the maps in one gmapsupp.img
One solution can also be to temporarily rename the gmapsupp-files to something else with the help of a PC or a smartphone.
And no, you do not have to use a special usb cable, the normal one will do.
If you want access to the files in the card from a PC the gps must be set in usb-mode but when transferring routes or maps from Mapsource to the gps this is not the case.
Confusing? You will get used to it, all thr tools and info (as well as maps..) are to be found in the forum!