ahh its been partitioned ok that makes sense
you can regain the space for main drive by deleting the secondary partition after you have moved all files to the new slave drive
it will be a bit of pissing around and if u get it wrong when deleting partitions all the files are gone so make sure you back it up before doing anything.

also if the computer is running a bit slow ect this could be a great opportunity to start with a fresh install and just use the new hard drive leaving the old one as it is for the moment (2 min job to swap the drives over) as you all ready have the external HD as a extra slave is not really that important as you all ready have space to backup