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I will report back when I check again but I am more weary of trusting moving a USB between computers than what should just be a single isolated private key. If other private keys on your offline wallet can become corrupted or damaged because you use a private key from that wallet it seems like a big issue.
The transaction that you generate on the online machine, is really just a text file... no binaries or anything like that... so if you have a nice new USB stick and the only thing on it is the transaction text file, then you should be fine to move backwards and forwards.
If both the offline and online machines are setup with "autorun" (and all that sort of nonsense) disabled, the risk involved of plugging in a USB stick and copying a text file off and then back on are pretty minor. Even the Armory Offline setup involves moving (un)signed transactions back and forth via USB.
On the otherside, I think I can see your point of view. I assume you are copying your Offline private key(s) out by hand, and then manually entering them into your Online machine. That does indeed take away the possibility of an exploit using the USB stick attack vector. However, the extra work involved with doing all this manually, plus the fact that it means you are having to manually track all your inputs and outputs seems to be a lot of work for very little real gain in security.