> It required re-uploading more than 300GB of data.

What should occur is that it must READ all of the files to make sure it has transmitted them already, which can take hours sometimes, but only a tiny, tiny amount of data is actually transmitted to the datacenter. The client basically shows endless streams of files flowing through it and saying "Currently Backup Up: puppy.jpg" but it isn't really transmitting the files, just verifying the contents haven't changed.

One way to realize it is doing this is watch a network monitor of some kind. Another is if it is going "impossibly fast", like you only have a 10 Mbit/sec upload pipe and it appears to be uploading at 100 Mbits/sec.