While sync can be used to restore a database in an emergency, sync should not be your only backup. The sync data is chunked, DEVONthink-specific raw data about the databases and is only useful to DEVONthink or DEVONthink To Go. A proper backup should be application-agnostic.

No, we're afraid. The official Dropbox API only pairs with one account at a time.

Not yet. If you would like to use DEVONthink or DEVONthink To Go with a service that is not yet supported, feel free to let us know. We might add support for it in the future.

DEVONthink for Mac always uploads complete databases. You can exclude indexed items, though.

DEVONthink To Go always syncs the metadata for whole databases but you can select for the location, the database, or individual groups if you want to sync their content files too. If you sync only the metadata you can download the actual content later on demand.

If you set an encryption key for a sync store all data is AES 256 bit encrypted before leaving the device and is stored in encrypted form on the server. It can only be retrieved using the correct key. The same is true for direct connections where all data is encrypted using the key set in the Bonjour options.

In addition, the username and password you set for newly created databases is used as soon as you sync the database to another device. The database cannot be retrieved from a sync store or via direct connection without entering the right credentials.

Unfortunately no. The encryption key is never shared with us. If we could recover your encryption key anyone else could do this too. We're using strong encryption to ensure that your privacy (at the time of this writing) can't be cracked without substantial computing power, if at all. Are you sure that you don't have noted it down on some piece of paper somewhere?