ERROR: Backblaze could not create a read-writable '.bzvol' directory on that hard drive

Why does Backblaze error with "Backblaze could not create a read-writable '.bzvol' directory on that hard drive." when I try to select a drive for backup?

Backblaze uses a hidden folder named .bzvol at the top level of every drive selected for backup. The directory contains a small file to uniquely identify the drive.

If Backblaze can't create a .bzvol folder, it will produce said error.

Common causes:

Disk Formatting:

If your operating system can read the drive but can't write to it, it will produce this error. Most commonly, this applies to Mac OS X and NTFS drives. Transfer the files to another drive, erase the disk as Mac OS Extended (HFS+) and put the data back on. Then it can be selected for backup.

Permissions (Windows):

• Show hidden files if they're not already shown: https://support.microsoft.com/en-us/windows/view-hidden-files-and-folders-in-windows-97fbc472-c603-9d90-91d0-1166d1d9f4b5 
• Open the drive's top level in Windows' Computer area
• If a .bzvol folder exists, delete it. Close the drive's top level window.
• Right click on the drive itself, and choose Properties -> Security
• See if SYSTEM is listed. If it's listed, make sure it's set for Full Control. If it's not listed, or isn't listed as having Full Control, click Edit
• Click "Add", enter "SYSTEM" (all caps) in the Object Names field, and click OK
• Make sure SYSTEM has "Allow" checked for all columns. Click OK, and OK again.
• Reboot, then select the drive in the Backblaze control panel

Permissions (Mac):

• Locate the drive in the Finder
• Click on the drive once to select it
• Pull down File and choose Get Info
• In the Sharing & Permissions section, click the lock icon and enter your Mac's administrator credentials
• Ensure that system or me has Read & Write privileges
• Select the drive for backup in the Backblaze system preference