store_with Dropbox do |db|
  db.api_key    = 'my_api_key'
  db.api_secret = 'my_api_secret'
  # Dropbox Access Type
  # The default value is :app_folder
  # Change this to :dropbox if needed
  # db.access_type = :dropbox
  db.path       = '/path/to/my/backups'
  db.keep       = 25

To use the Dropbox service as a backup storage, you need two things:

The default db.access_type is :app_folder. This is the default for Dropbox accounts. If you have contacted Dropbox and upgraded your account to Full Dropbox Access, then you will need to set the db.access_type to :dropbox.

Note: You must run your backup to Dropbox manually the first time to authorize your machine with your Dropbox account. When you manually run your backup, backup will provide you with a URL which you must visit with your browser. Once you’ve authorized your machine, Backup will write out the session to a cache file and from there on Backup will use the cache file and won’t prompt you to manually authorize, meaning you can run it in the background as normal using for example a Cron task.

Chunked Uploader

The Dropbox Storage uses Dropbox’s /chunked_upload API. By default, this will upload the final backup package file(s) in chunks of 4 MiB. If an error occurs while uploading a chunk, Backup will retry the failed chunk 10 times, pausing 30 seconds between retries. If you wish to customize these values, you can do so as follows:

store_with Dropbox do |db|
  db.chunk_size     = 4 # MiB
  db.max_retries    = 10
  db.retry_waitsec  = 30

Note: This has nothing to do with Backup’s Splitter. If you have a Splitter defined on your model using split_into_chunks_of, your final backup package will still be split into multiple files, and each of those files will be uploaded to Dropbox.

Also note that in Backup versions prior to 3.3.0, the Splitter was required to upload files to Dropbox that were larger than 150MB. This is no longer the case. You may still use the Splitter, and you may now split your final backup package into chunks larger than 150MB.