FIX: LaCie Thunderbolt 5Big will not mount with High Sierra
Ok, I **HAD** the same problem (LaCie Thunderbolt 5Big would not mount with High Sierra).
Seems I've fixed it for me anyway. Here's how:
Download LaCie Raid Manager install package from LaCie Website
Reboot your mac in safe mode (Hold down the shift key while it reboots)
Goto Applications folder. Find "LaCie RAID Manager" folder, and use "Uninstall LaCie RAID manager.app" to uninstall Raid manager. (NB: this will not work if you have not started up in safe mode).
Once you've uninstalled LaCie RAID Manager, use the LaCie Raid Manager install package you downloaded earlier to install the latest Raid manager.
Reboot normally.
Plug in your LaCie thunderbolt drive and wait for it to start up and mount.
I sure hope this helps out a whole lot of people because I just hate it when things don't work after an upgrade, and I hate spending time figuring out how to get back to where I was before the upgrade. I've had a lot of help from the internet over the years - here's my little bit of paying back. At least, I hope it works for you.