BC as default .gpx AND how to locate and share my baseline maps

Maps are in the Garmin folder. But you can't 'share' Garmin maps ... except the base map. That is named gmapbmap.img

In Windows 7 Basecamp is at \Program Files (x86)\Garmin\BaseCamp\BaseCamp.exe

What if my micro SD card - which presumably has maps on it - was taken out of my GPS and put in my friend's computer? Would he be able to use Base Camp to download the maps and then transfer them to his GPS?

...and also, is that gmapbmap an .img file?

It depends on the maps and how they got on to the card. If, for example, you downloaded City Navigator and put it on a card, it would only work on your own GPS due to copy protection. Same thing if you purchased City Navigator on DVD and installed it on a card. If you purchased City Navigator on a pre-loaded card from Garmin, the card can be used in any GPS but it cannot be duplicated.

Other maps, like the free ones from GPSFileDepot or OpenStreetMap might be transferable, but it would be better for your friend to just download them directly to his own computer since they're free anyway.

When an .img file is listed as a "disk image", that is bogus. Unfortunately the .img file extension is used for several completely different things and Windows is making the wrong assumption about what it is. If the .img file comes from a Garmin GPS, it's a map. gmapbmap.img is your basemap. There would really be no point in "sharing" this because it's a really crude map with very little detail, and it is already provided on all Garmin units.

Exactly what maps are you trying to share?

The maps I'm trying to share are all topographic maps from free online sources. I guess I was wondering if "unlocked" and non copy protected maps could be shared from device to device without having to do much footwork. From what you're saying it shouldn't be too much trouble to take the maps from the micro SD card, transfer them to the destination hard drive and then load them into BaseCamp for transfer. It's unfortunate there isn't a simple and transferable format, a .gpx of the map itself, which the computer isn't going to assume is a CD!!! But it seems BC is able to "decode" .img files and thus make them transferable. And, that failing, MapSet Toolkit.

You're gettin unnecessarily complicated here. If you have free topo maps working on your sd card they will be files ending in .img. You can't 'load them into Basecamp for transfer' and indeed there is no need to. They can simply be copied from one device to another without issue, or as Boyd points out just downloaded again directly by your friend.

If (for whatever reason) your friends don't want to download the maps again, you can use the backup/restore features of GMTK to transfer them to a USB thumbdrive and install on their computers: http://www.javawa.nl/gmtk_en.html

If your computer thinks .img files are CD disk images you could go to Control Panel > Default Programs > Set Associations and choose the .img extension. You could then click the browse button and choose either Mapsource or Basecamp. Now, you can't actually "open" an .img file with Mapsource, but after making this change then any files with .img extensions will be listed as "Mapsource Applications" and that should prevent the computer from thinking they are disk images.

BC as default opener for .gpx files would be great, but it is a feature that the devs need to add.

BC as default opener for .gpx files would be great, but it is a feature that the devs need to add.

You can already make BaseCamp the default program for opening .gpx files.

If (for whatever reason) your friends don't want to download the maps again, you can use the backup/restore features of GMTK to transfer them to a USB thumbdrive and install on their computers: http://www.javawa.nl/gmtk_en.html

When we're talking about about precompiled img files from devices JaVaWa GMTK won't help; GMTK is meant for managing the maps that are installed on your computer. One of my other programs, JaVaWa Device Manager, was created for managing devices, but the backup/restore function of JDM isn't suited for transferring these maps (the backup/restore function is limited to the same device to avoid problems with commercial maps).
Transferring maps isn't rocket science tough, just as SUSSAMB explains. The only important thing to keep in mind is to put the img files in the right folder (on the outdoor devices it's always "Garmin"; on the device itself or on the SD card).