Software to sync data between PCs with usb storage?

Good evening,
I am looking for inexpensive software that runs in portable mode on a usb key that would allow me to keep data on 3 separate pcs in sync. My problem is that only one of the PCs is physically connected to the internet, and the folders I need synced are too large to feasibly be put on a usb key. For example, my data and graphic files for a project I'm working on is 33GB, but my USB key is a nominal 32GB.

I would like to run a sync program on the main pc, then take the usb key to the other machines and have it analyze for differences so when I go back to the main PC it can copy or mark for deletion only files that changed on the main pc. After that point, I would be in blissful sync glory.

Does anybody have any ideas or suggestions on software that might accomplish this?

Until recently I could just keep a copy of everything on the memory key and sync to and from the key itself (MainPC -> USB, USB -> test machines)
Thank you for any help.

Sorry, no software recommendation. Such an app probably doesn't exist, though it's possible. I did look around because it's an interesting concept.
Didn't find it, maybe because storage is so cheap that there's no demand for it.
It may or may not be complex to develop, and need a step to delete synced data from the stick so it doesn't fill up, yet keep all file info needed for syncing.
That's the main issue I see, but I'm no expert on file parameters used for syncing.
Using KISS, it's best to have all the actual data you want to mirror.
That also provides an additional backup.
I suggest you get a 64gb stick or enough USB HD or SSD and use the software syncing method that has worked for you.
But maybe somebody can better answer your question.

Thank you for your reply Victor.
I have been thinking about a larger storage device as you mentioned. I would rather have half of my project stored on a portable device though, so if it were lost it would be some graphics files and some incomplete code that would be basically useless to whoever found it.I have tried tools like Richcopy to only copy what has changed since last approximate update, but it seems to try and copy everything. Maybe I'm doing it wrong.