I liked SpiderOak a lot for its security, but sadly, it was never very reliable as a backup or sync service. I spent more time troubleshooting and rebuilding databases and clearing serverside caches and so forth in 8 months there than I have in almost 5 years of other services. And because of that, I never really felt like I could trust it as a backup.
Also, realize that SO is a poor substitute for Dropbox, as it's only really a backup service. Although it includes a sync option which works passably, the moment you access your files from a mobile device, you upload your keys to the server, and zero-knowledge goes poof. And even if you're willing to give that up, I've never met a mobile app which allowed you to access and manipulate files in the ways Dropbox does. You can do some basic "share just this file" stuff, but it's a very different product.
I really hope that was just because I was on it when they were growing massively due to the NSA leaks, but sadly, for me, they were a textbook example of "Great idea, bad execution".
> Another benefit of SpiderOak is that they de-duplicate your data, so you may end up using less space on SpiderOak than you do on your own machine.
This benefit soon disappears due to two reasons:
1. If your files change, SpiderOak saves versions of them indefinitely, with each taking up space (of course, all the deltas and stuff). There is no simple way to set it up so that file versions older than X days/years get deleted. The SpiderOak client is completely useless if you want to delete older versions of files because you'd have to wade through all your directories and subdirectories looking for files with multiple versions.
2. The SpiderOak client has also been buggy and ends up creating file versions of unchanged files too (like photos you may have saved once on your computer and never touched). So there's no easy way for you to get to specific places where files have several versions stored and do some cleanup.
This cleanup of older versions matters a lot more if you have a smaller account quota, lesser free space on your account, and you're not willing to pay for the $129 a year 1TB option that is heavily pushed (compared to the other tiers) by skewed pricing on SpiderOak's part.
Overall, SpiderOak is still better though. Dropbox is more dishonest in its approach to deduplication. Dropbox dedpulicates data across user accounts (so if you and I store the same free eBook from Project Gutenberg on our Dropbox accounts, Dropbox saves only one copy) and deduplicates data within your user account for files you may have replicated across folders. But in both the cases it treats your space quota as if deduplication is not done at all, effectively charging you for more than the space you're actually using.
Dunno about SpiderOak, but the way Tarsnap does it, is that as blocks are encrypted and uploaded, the client keeps metadata about them locally (presumably an hash, size, etc). Then that metadata is also encrypted and uploaded to the server. When it wants to upload more blocks, it just looks at that metadata and skips duplicated blocks, updating only the metadata to point to the existing block.
You can share files with other people, similar to the way you can with Dropbox. That might indicate that encryption is done per-file (which is actually a little less secure, so who knows).
I haven't researched it, but it could work like this: scan the local machine, find duplicates, upload unique files, and then create links to any place a file is duplicated. It all happens locally, so only encrypted data is ever uploaded. Some tiny bit of info about the structure of the file system might be transferred and known by SpiderOak, but I can't conceive of a situation where that matters.
