If the drive isn’t recognized after a reboot, it may be that the drive was manually powered off.
The LaCie d2 Thunderbolt drive won’t power on if it was powered off by the user pressing the power button previously.
When the power button (blue LED) is pressed for 5 sec it will turn off the d2. The drive will stay turned off until the button (blue LED) is pressed again.
If the computer is rebooted then the drive will not mount on the Desktop or show up in Disk Utility.