If you attach an Elgato Thunderbolt™ SSD or Elgato Thunderbolt™ Drive+ drive to your computer, Microsoft will think that your computer setup has changed, and will ask you to re-enter your Microsoft Office product key.
This is a known issue if you own Elgato Thunderbolt™ SSD or Elgato Thunderbolt™ Drive+, another Thunderbolt™ drive or an eSATA drive.
This will happen each time you plug in a Thunderbolt™ drive or an eSATA drive.
FeedbackElgato Thunderbolt™ SSD or Elgato Thunderbolt™ Drive+ owners can send feedback and requests for Microsoft to fully support Thunderbolt™ and eSATA.
This Microsoft article has general Office for Mac product key troubleshooting: