For very important projects, one backup in the recording studio, one backup in a storage unit, one backup overseas. Plus Acronis image of the studio computer C drive which is separate from the projects drive. Plus backup of the plugin installers and Kontakt Libraries, same as above. The only issue with this is, keeping them up to date. Only the one in the studio is most up to date all the time, the rest are delayed. Also, I usually keep multiple copies of any albums while I’m working on them, especially if the client is just paying as they’re going or putting a deposit down at first. Having an album disappear before any of the songs are finished or mixed would be a big problem. I keep them at different locations.
I’ve used to use SugarSync to sync over multiple devices. Until, I disabled the sync on my laptop and deleted the files due to running out of storage space, then I ran into an issue and had to put the laptop back to its previous state with Acronis. At that time, SugarSync was enabled again, decided I deleted the files since they were no longer there and remotely deleted/wipe the files from the studio computer and home computer as well as from their own cloud server. That is the only time I ever lost I file. I thought it was gone forever until I found a hardware backup. Now I hate all syncing services. Hard drives all the way. Syncing is not backing up. If one computer gets corrupted, they all get corrupted. Same if the cloud computer server holding your files gets corrupted or goes down, all your files on all of your devices get wiped and deleted. It’s actually riskier than just leaving it on one computer.