@IrwinElectronics
Thank you for your question.
Data is divided into chunks, that is also what the deduplication is based on. (As far as I know you can even change the chunk size yourself.) Borg creates chunks of parts of data and whenever new data is in part consisting of such chunk, the chunk won’t be written down additionally, as it is already there. The chunks are small in size and there will never be just one huge backup file with Borg.
Another plus with Borg is that it uses chunk indexes to speed up backup processes. While Borg is not needed on the server side to save data, it is very recommended as it speeds up the backup process immensely by using aforementioned chunk indexes. Depending on the type of data and amount of changes before each backup, it can sometimes take maybe a minute or less to backup huge amounts of data, if there are for example little changes.
You could alternatively back up to your primary and secondary drive during each initial backup instead of chaining them. However I assume you want to keep the backing up at night property of your current setup, so you could also use Borg to back up your initial backups at night, which would also save a lot of time in the long run.
I don’t know CrashPlan so I don’t know how it works, but if it, similarly to rsync, copies only modified files (I would go for the safe route and check against checksums, not metadata) then it should be able to copy only what’s necessary properly.
As explained, everything is first divided into individual chunks, then further operations like compression and encryption are applied on these chunks. So size-wise you could assume them to be “files” except none of them will be bigger than the chunk-size you set in the first place.
I found the following, which provides information on backup mirroring:
https://borgbackup.readthedocs.io/en/stable/faq.html?highlight=chunk#can-i-copy-or-synchronize-my-repo-to-another-location
In terms of chunk-size I found this:
This at --chunker-params:
https://borgbackup.readthedocs.io/en/stable/usage/create.html?highlight=chunk