imac crashes when connecting external drive thunderbolt 2

Hi,

I've just purchased a new imac 5k retina 4ghz etc. After a very rocky start with it. It's up and running well, until I plug in my lacie d2 thunderbolt 2 drive. If I connect it via thunderbolt it crashes the imac, but if I connect it via usb3 it's ok. I'm guessing this is a thunderbolt issue? I'm wondering if anyone else has experienced this,. I saw a thread for issues with thunderbolt and 10.10.2, but it suggested 10.10.3 (which I'm running) solved the problems, but this is not my experience.
Thank you for any suggestions.