well I know a bit about this. lets see If I can help:
This is they way it goes.
Your Hard-Drive is split to partitions (You prolly know this already, but I found that the best way to explain something to someone is to start at the begining and talk to them like they are 4yr old
On a partition there is a file-system.
Depanding on your OS and what you choose when you installed.
You have FAT32 (for linux) and NTFS (for winblows) (NTFS was made by M$... it has some pluses... but they won't reveal how it works. That's why you can't write from linux TO an NTFS partition... it will mess it up. But you can read and copy files FROM it to the FAT32 partition)
You can either ask a friend for a 100GiG hard-drive to be loaned to you for one day... (format it as FAT32, transfer the files from one of your 100GIG drives, re-format your now empty hard-drive and transfer the files back to it from your friends hard-drive, and do it again for you second one...)
or do what I said before.
first of all, only keep files that you REALLY have to keep.
If you have movies or stuff like that burn them to a cd (if you can) and delete them from the hard-drive...
After you clean-up, compress, and defragment your hard-drive, the files on it should take a lot less space...
When you defragment your files are moved to the begining of your partition.
remember I talked about file-systems... they are only diffrent in how they place the files on your partition.
well, imagine that your partition is a big white block, and the file-system devides it to small blocks. your files are stored in the small blocks.
A block with a file is black... well before you defragment, they are all over your partition.
After you defragment, all the black blockes will be closer togther and on the start of your partition... get it?
If there are files in the MIDDLE of the disk, then the defragment process didn't work on them for some reason. if you will delete that slice you will lose those files... that's all.
try moving files from on of the 100Gig hard-drives to the other and then doing the steps I said again... maybe that will work...