As far as I know, Backblaze lets you exclude folders from accessory drives and so forth, but not from the main PC drive itself. Also, the selection by exclusion itself is deeply annoying, because, as I said above, if out of many folders you only want to back up a few, selecting just those would be much easier and more intuitive than deselecting a whole pile of them to only leave a few. This was a deal breaker for me.

I definitely have folders excluded on the main drive.