Shoutbox

Your question is somewhat unclear, so I am just going to state what you can do.

1) You can format the previously unused hard drive and assign it a new drive letter.  For example, if you already have drives C: and D:, it would become E:.  It sounds like that is what you don't want to do.

2) You can format the previously unused hard drive and map it as a folder under an existing drive.  For example, it could become C:\Music.  Note, however, that doing this doesn't truly merge the drives, it just links them under that path.  You will be able to put 250GB worth of files in the linked folder and its subfolders, which will go on the second drive.  However, everything else on the first drive will remain constrained to its size, so this is not quite like having a single 500GB volume.

3) The only option that will give you a fully functional illusion of a singe 500GB volume is a RAID-0 stripe set.  There a couple ways to set this up, either through Windows or through RAID support on your motherboard, but none of these are non-destructive.  You will have to back up the data you want to keep and reformat.  There is another drawback to choosing this option -- it doubles your chances of losing data to a hard drive failure, since now you have two drives that can fail, either completely destroying the integrity of the volume.

Think it over and respond by posting which you want to do, and I am sure you will receive more specific instructions after having it narrowed down.