Backblaze was designed to be lightweight. There is a small process that slowly scans your computer looking for new/changed files. It can take about an hour or two to scan across all drives for an average user. When you plug in an external drive, it may take Backblaze anywhere from a 1-120 minutes to schedule the files on the external drive to be backed up online.
There is a way to force an immediate rescan. It will slow your computer down for a few minutes while it scans all your drives. You can force the rescan by holding down the OPTION key and clicking "Restore Options..." button.