If your LaCie drive is not mounting on your Mac, try these fixes:
Reconnect your drive.
Restart your Mac.
Force Mount your drive using Disk Utility’s Mount tool.
Repair your drive using Disk Utility’s First Aid tool.
Reformat your drive.
Warning: Reformatting your drive will wipe the data on it. We suggest cloning your drive and using that clone as the target for data recovery tools. Disk Drill has both backup and recovery functions built in, so we recommend using it for this task.