Linux Mint and lost hard drive space

Newbie here. I posted this in the Linux Mint and DBaN communities. Posted this here too because I just wanted to check all my bases. More information is better, right?

I was wondering why Linux Mint 11 (is this a problem for other distros too?) seems to eat up so much space on hard drives, both internal and external - and if there is a way to access the missing space that doesn't involve wiping everything or a lot of tweaking short of taking everything apart.

I'd installed Mint on a 320 GB hard drive (had my eeepc 1005HA stock 160 GB hard drive upgraded, along with the memory, if that's important), and found I only had around 270 GB of hard drive space left. And when I reformatted my 1 TB external hard drive (NTFS to ext3/4), I had only 850-70 GB left, more or less. I'd decided to DBaN the internal drive, but DBaN read the drive as a 298 GB one. I did a Quick Erase anyway, and when I booted to DBaN again to see if things were all right, the hard drive still read as 298 GB. (Is there something really wrong if even DBaN doesn't read the entire drive?) But when I reinstalled Mint, it was read as a 320 GB one again.

By the way, I called the service center where I had my computer upgraded and they said that there were no problems with the hard drive, that the BIOS really did read all 320 GB of it.