I'm a little confused - the 250GB freeagent says 0 bytes free.. but then you mention the computer, then how much space you would need.
To check how much is being used, go into 'My Computer' ('computer' on vista or windows xp), and it should list the drive with the Total Capacity, Available Space, Used Space.. if you're trying to find out what's using space, you can open the drive to see the contents (you might have a backup program that just kept backing up until it ran out of space?)
150 pictures shouldn't be more than 1GB or so (even at high-resolution RAW files), so something is wrong.
As for finding out how much space you need, it's a bit tricky as all the windows system files will count as 'used space' on your main drive, and you won't want to back that up (just the content like videos, pictures, files, etc..)
What are you trying to do/wanting to backup?
