The .bzvol is a unique identifier file written to the root of each secondary hard drive selected for backup and within the Backblaze installation on the primary hard drive. This file is used to uniquely identify each hard drive selected backup.
If you are receiving a warning from the Backblaze software about an issue with the .bzvol missing from a hard drive, these steps will also need to be completed:
1. Open the Backblaze Preferences pane.
2. Select the 'Settings' button.
3. At the bottom of the pane are the drive selection settings. In those settings, two entries for the hard drive will appear, one selected with the (unplugged) status and the second unselected. Select the currently unselected entry.
4. You may receive a message that this action will replace the "old" drive. Confirm the selection and allow the drive indexing to complete. Replacing the “old” drive will not affect the backed up data. Your files will go through a process called ‘deduplication’, where the location of each file is updated: https://help.backblaze.com/hc/en-us/articles/217665548
5. Select the 'Ok' button to confirm the settings changes. When convenient, reboot the computer. After the reboot, only a single entry for the hard drive will appear in the Backblaze settings.
The Backblaze software will require some time to reconcile with the existing backup, after which backups will resume normally, as scheduled.