[Geoff Bacon]

You could test using a USB cable with the Companion to see what sort of time the transfer takes (probably faster than wifi)

I see you have tried backing up to dropbox and are awaiting a response to having no backup file afterwards.
As a matter of interest, how long did that backup take?

Try using a different service (google drive?)

Might be worth keeping a copy on one of these cloud services and synching from the tablet to the cloud (keep it one way)
As this doesn't preserve your files if something goes wrong with the sync (you could have a damaged database and an identical damaged backup), occasionally take a full backup as well (I know that is what you are tryig to do now!)

Geoff