Hi. Further to alexdelalane's solution, I have found that if you want to use the Yosemite icon for Time Machine (or any other OS X version-specific icon), instead of finding an icon on the web, the easiest solution is to:
1. "Get Info" (command-i) on any drive that is currently displaying the Time Machine icon.
2. In the upper left, click on the icon image so that the blue "glow" around the icon appears.
3. Copy the icon to the clipboard (command-c)
4. "Get Info" for the drive displaying the generic USB drive icon that you are using for Time Machine.
5. Click on the icon image in the upper left so that the blue "glow" around the icon appears.
6. Paste the icon on the clipboard to the Get Info window icon (command-v).
7. Go back to the "Get Info" window for the original drive that you copied the icon from (the one with the original "correct" icon).
8. Select the icon in the upper left and then paste the still-in-the-clipboard icon onto the "good" icon - even though it has the Time Machine icon.
This replaces whatever the system is assigning, icon-wise, to the drives with a custom icon - in this case the same system icon that you want to use. I have found that the Time Machine icons that I have pasted into the drives have remained in place using this method.
If you decide that you want to revert to the original icon state for these drives, that is, you want the system to resume deciding what the icon should be, all you have to do is go back into Get Info, select the small icon, hit "delete" and the original system icon (which cannot be deleted) will return.