Assuming that your LaCie is connected via USB, take a look under the USB tab in System Profiler or System Information (whichever it's called in Sierra) and see if it's recognized.
Either the drive or the port itself may be bad. Have you got another device you can try in that port as a test?
The new Great Equalizer is the SEND button.
In Memory of Harv: Those who can make you believe absurdities can make you commit atrocities. ~Voltaire
