The best way to handle that would be to set up a separate physical
partition for the sync drive, and add the "physical" drive to Vbox.
I believe you can also have the drive mounted in the host OS
simultaneously. But there may be more hoops to jump through.
Another option is to sandbox the client.
I think the only way to get both server and client to function on the
same machine is to build them from source. However, I have only seen
with no SSL support on the server, which then breaks Windows clients,
which require SSL on the server.