It is the Finder's No Write icon. It is stating that you are not permitted to write to a general location that is write protected, until you specifically choose a location on it that is writeable by you — if one exists.

For instance, the boot drive name in the Finder sidebar has this setting because you have not specifically chosen a permitted, writeable location on the drive (e.g your home directory). This icon also appears with my Synology DiskStation network attached storage (NAS), until it is mounted, and I have selected a specific folder on it that is writeable by me. Same story with a mounted USB drive.