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If you want to use pre-Thunderbolt 3 peripherals like LaCie Rugged Thunderbolt External Hard Drive (USB 3.0) on Lenovo's ThinkPad P50 Mobile Workstation Laptop, you need to use special adapters like the StarTech Thunderbolt 3 to Thunderbolt Adapter.

First use the connect the LaCie Rugged Thunderbolt External Hard Drive (USB 3.0) to the StarTech Thunderbolt 3 to Thunderbolt Adapter through the Thunderbolt 2 based mini-DisplayPort connector..

Once you plug the StarTech Thunderbolt 3 to Thunderbolt Adapter into the Thunderbolt 3 port of the Lenovo ThinkPad P50, a message window pops up on the bottom right corner of your screen, telling you to enable the Thunderbolt device before use. Note, you will see the Thunderbolt device that is connected to the StarTech Thunderbolt 3 to Thunderbolt Adapter as the device that needs to be approved (and not the adapter itself).

After selecting 'Always Connect" in the Approve Thunderbolt Devices window, you can see the LaCie Rugged Thunderbolt External Hard Drive (USB 3.0) show up in the Attached Thunderbolt Devices window.

To our surprise, LaCie Rugged Thunderbolt External Hard Drive (USB 3.0) did not enumerate on Lenovo ThinkPad P50's Device Manager. 

We made sure that the laptop was up-to-date with the latest Thunderbolt Firmware. Unfortunately, the StarTech Thunderbolt 3 to Thunderbolt Adapter does not work well with the Lenovo ThinkPad P50, which means you can't connect to pre-Thunderbolt 3 peripherals like the LaCie Rugged Thunderbolt External Hard Drive (USB 3.0).