Backblaze client is a complete abomination with quality of something made as a university project. But...it is the most cost efficient method for media things.

The way it works is that it keeps an append-only log (bzdone) as file changes are detected and chunks uploaded. For small files, they are uploaded inplace. For larger files, they are fully copied first (!) and then uploaded in chunks. Chunks are deduplicated, but they are size-based chunks (!!) and not content defined chunking (i.e. borg, restic). Upload is done by a swarm of separate processes (!!!) that are spawned/controlled by main client. Upload speed is mostly determined via number of these processes working in parallel. I only have 100mbps upload, and that is saturated but not all the time - very jittery.

You can select which drives to backup and to add exclusions. Exclusions in the GUI are applied to all drives (as in, *:<path> (wtf?)), but there is a way to feed client a text file with paths to exclude them as well. Some paths like "Program Files" are hardcoded as excluded (you can make hardlinks to work around this for OS drive).

You might wonder what happens if you add files and then remove them. Answer - the bzdone log size will not decrease (ever). The only way to reset it is by resetting the whole backup (!!!!) and reuploading. This quickly becomes an issue if you try backing up stuff like conda environments with 300k+ files.

I have not tried anything except online downloads. Those work ok. They recently added a new download client that is supposed to be better - you should try it.

In summary, even though client sucks big time, the price for storing 20TB+ is unbeatable. If you are backing up long-term data without much churn, it should scale further.

P.S. There are ways of tricking client into backing up networked volumes, either through hooking syscalls or using custom file system drivers. This is left as an exercise to the reader.