Freecom Hard drive. I have a Freecom 400GB external HD to use as a backup to my PC drive C. My PC

Question

I have a Freecom 400GB external HD to use as a backup to my PC drive C. My PC drive has used around 27Gb of its 40Gb capacity, yet when I run a backup to the 400Gb drive it only shows about 6Gb in total space has been used. All my files and folders seem to backed up OK, and indeed I can open them from the 400Gb drive with no problem, but I cant see the Windows files as shown when drive C is opened. So how can 27Gb be backed up into a space of 6Gb. I dont understand. I use XP Pro SP2 and all updates.