How to split a garmin unlocked map

Hi everybody,

I've been trying this for a while but with no success so far. The thing is I have this huge 1GB unlocked .img garmin map loaded in Basecamp and I would like to take just one region and save it as another .img file, so I could install it easily on my android mobile and use it with Streetpilot. When I try to install it on my USB drive, it says "No unlocked maps were found for this device."

Is there any way how to make this happen?

Thank you in advance!