I have a drive with three partitions: C: and G: and a hidden recovery partition. Windows recognizes the G: partition as a removable drive (judging by the icon). When I try to use the built-in backup tool, the drive is available only as a destination for the backup, not a source (backing up to a NAS).
Screenshots:
The computer was upgraded from Windows 7 Home to Windows 10 Home in order to be able to backup to a network location. The same icon appeared in Windows 7. I don't remember when the icon appeared.