This seems a use case for dm-userspace+cowd: in essence, you would set up a DM target (block device) consisting of a COW (copy-on-write) file and the block device corresponding to your USB stick, and use it to host a filesystem. All updates would go to the COW file; reads which are not in the COW file would be served off the USB stick; after you unmount the filesystem, merge modifications from the COW file into the USB stick.
Unfortunately, it's Linux specific and development seems to have stopped in 2007.
If what you want to do is sync'ing files across two (or more) PCs, may I suggest that you put your home (or relevant folders) under a versioning system?put your home (or relevant folders) under a versioning system? The usual work cycle becomes like this:
- plug in USB stick;
- update home directory repository by pulling latest changes from the USB stick;
- do your stuff;
- commit changes to the versioning system and update repository on the USB stick.
That's only one write to the USB stick. (Although I agree with what others have said, that by the time yoru USB stick wears out, you will probably have bought another -larger- one.)