SyncToy is a good utility (and free). But every-so-often i’ve found it to hang. It will be on one particular sync and no matter how many times I’ve tried, it will not budge.
After much playing the solution is to remove the various oddly named .bin files in the SyncToyData folder in My Documents.
Dunno why and can’t find any reference on the ‘net to anybody else having this issue.