Quote:
rsync is highly regarded.

Highly regarded for network copies over a relatively (to the CPU speed) slow connection, with intelligent/fast CPUs at both ends of the link.

Lousy choice for use with a Zip disk, though. Rsync requires reading/summing the existing data as well as the new data, so it will have to do slow reads from the existing data on the zip disk, which would only slow down this process.

Cheers