I have been using sync for years to copy data off my computer onto an external drive. This worked fine and would copy files over if they were new ore more recently modified at the source (and not change the target in any other way.)
With the update to Toucan 3.1.7 I have a lot of messages indicating my target directories had been removed (which as per help should just not happen in synch mode.)
a) if this is a bug (and if so when will it be removed)
b) whether I should be using a different mode to create backups off my current source to a target that has the same structure as the current source
I just started to try out Toucan with the 3.1.8 version. Those were most of the messages I was getting as soon as I started the problem. I am in Sync --> Copy mode and as you say, the documentation clearly states that nothing will be deleted in this mode.
Can someone help us with this problem?
Toucan's developer is a member here and he is probably the best source of information you can find for it.
Give him some time and he should respond.