You can hold the ⌥ option key, click on the menu, and then click System Information. Highlight the USB under Hardware and on the right side to see if the LaCie drive is listed in the USB Device Tree. If it does, then the time machine backup drive works fine. You can drag the drive icon on the Mac desktop to Mac Trash, then unplug the drive. Note the unplug will discontinue the Time Machine backup job.
If the Time Machine backup drive is not listed in the USB Device Tree, I guess you need to perform some solutions to make the drive functional again. Please read this article for detailed solutions: Fix External Hard Drive Not Showing up or Recognized on Mac
