As you can see in the images the icon for the external drive Quetzalcoatl has update to the Yosemite version, however the Time Machine disk hasn't. Is it possible to force the use of the new icon?
Edit: There is nothing wrong here.
Under /Volumes/, there are two type of mounted volumes associated with Time Machine:
First is the hard drive the backup sits on. If you use Time Capsule, the hard drive inside is by default called Data. Because it's access through network, the volume in the picture has a Shared Disk icon. If you use an external drive through USB, the volume would have a USB/External icon.
Inside, there is a file with name like Someone's Macbook Something.sparsebundle. When backing up, Time Machine would mount this file and create /Volumes/Time Machine Backups. This volume is covered with Time Machine icon, and it only appears with TM is running. The name cannot be altered as far as I know
My conjecture is that Budda in question is a volume of a hard drive, not the mounted volume of the sparsebundle. Like other files, volumes can have its icon changed through Get Info window, which is probably why a generic disk got a TM icon in the first place.
Cut and paste the new icon from the location below, and you will have this resolved.
I see the incorrect icon displayed every so often with Yosemite and El Capitan.
This is always fixed for me by relaunching the Finder - one way of doing that is holding down Alt, then right clicking on the Finder icon in the dock, and then clicking relaunch. After that the correct icon is shown on the desktop.
You could grab the right one from here & paste it in...