When I noticed this evening that the amount of data listed as backed up by Backblaze was much smaller than it had been previously, I did some sleuthing. I found some very important information in the external drives help pages (emphasis mine):
Backblaze works best if you leave the external hard drive attached to your computer all the time. However, Backblaze will backup external USB and Firewire hard drives that are detached and re-attached as long as you remember to re-attach the hard drive at least once every 30 days. If the drive is detached for more than 30 days, Backblaze interprets this as data that has been permanently deleted and securely deletes the copy from the Backblaze datacenter. The 30 day countdown is only for drives that have been unplugged. There is no countdown for local files.
If you, like me, have something important on an external drive—like all the photos you’ve ever taken, or something like that—be sure to plug it in regularly. You might want to make a recurring alarm to remind you. Just thought you should know.