Typically, Finder displays the Eject symbol next to an external storage device, and an inverted version during mouseover, like this:
but today, Finder displayed a Refresh symbol, and an X during mouseover, like this:
The storage device was my Time Machine backup and was connected via USB. The device appeared normal in Disk Utility. I did not interact with the symbols, but ejected it using Disk Utility. After replugging the device, the unusual symbols did not reappear.
Can someone link me to documentation regarding the meaning of this (preferably on Apple's Support site)?