Brian from Backblaze here.

The focus of our backup system is being "easy to use". We get a lot of advanced users who are computer savvy, but we also get some mom and pop novice who aren't very good with computers.

Ok, so we don't allow you to unselect your C:\ (main) drive. Here is the reason: when given that choice an alarmingly high number of novices accidentally unselected the drive. (sigh)

We don't charge customers any more or less on what they backup, so there are almost no legitimate reasons to UNSELECT a drive. What data do you have that you hate so much you MUST lose it when your computer is stolen? So we error on the side of helping these novice users and don't allow them to unselect their main laptop drive. The down side is it sometimes irritates the advanced customers who know what they are doing. :-)

Yev from Backblaze here -> There are certainly items related to Backblaze that we need to back up. You can exclude pretty much everything inside of the C: drive, but we do require the drive itself to be selected. Also, we chose the "backup everything, and let the user exclude data" route because the other way seems like a gap. For example, a lot of backup solutions make you select what you want to back up, but what if you forget something? Or forget to add something later? That would leave a hole in the backup, so we go the reverse route!