Data swap application?

I have 2 external hard drives, both completely full. If I wanted to swap the data over (one is 500GB, the other 1TB) is there such an application (OSX) that will do that, block by block, rather than having to use a third drive to act as swap space?

[edit] before anyone starts flinging names at me, I've clarified below that I know the drives are different sizes, I mean I want to swap all of the data on the half TB for half of the data on the TB.

How can you swap them if they're both completely full? One is twice the size of the other!

I also don't get why you'd want to, TBH.

*confused* :P

What I mean is, swap HALF of the TB for ALL of the 500GB...

Was this a joke I walked straight into, or something? :/

[edit]Heh, trust me to post just as you were posting. :P

LOL - is this a trick question, or just a stupid one?

Hmm, maybe the name "mrtricks" is a clue...

No trick, and thanks, I'm not stupid. What I mean is this: I have a TB and half TB drive, both full. I want to swap all of the data on the half TB with *half* the data on the TB. Usually, I would need a spare half TB drive and it would take three transfers. I wondered if there is a program - maybe in the same family as those backup programs that come with drives - that can swap the data without needing more than a few MB of spare space, even if the files themselves are bigger.

no there is no such tool invented.

but as you have come up with the idea maby some one else also come up with the idead som maby there is a tool for swapiong sectors.

Why not only swaping the files ??

The drives are scratch disks for video editing, so a lot of the files are up to 13GB in size...

If your computer has plenty of RAM, then you could use some virtual disk/ramdrive type solution. I bet there are plenty of free tools out there. Google 'virtual disk' and you should be on the right track.

EDIT: Right, I got GB and MB mixed up. RAM is hardly sufficient for moving 13GB files...

the best way is to buy a cheap backupdisks.

that way you can store the backup files on it too.