Lew,
Maybe that's why the DOE facility I work for switched from DropBox to MS Box a year ago or so, though I'd be more worried about MS security than DropBox's just on general principles. Some folks at work (not me!) work on nuclear threat issues so I'd think security would be a serious consideration for them. IAC, I doubt anybody is after my work files, or the personal files, that are located on DropBox. That's not to say others like you should feel the same way, only that for users like me there is a benefit to cloud storage. For me that benefit is invalidated if there is no transparent way to keep the files on the local hard disk but maybe OnShape addresses that issue. Alas, I lack the expertise or interest to set up a system like you have and mostly satisfy myself by periodically backing up to an external hard disk.
Looks like OnShape is being very discrete with details on their product so it will probably be a while before real details are made public.
Mike