So after rebooting my iMac Pro, it no longer mounts my Shuttle XL. I have rebooted 3 times now, and NOTHING (not showing up in Disk Utility). G-Speed studio software sees the RAID array and can configure things on my drive. My MacBook Pro can mount the drive. It’s just this iMac Pro that can’t mount the drive.
UPDATE: John D from G-Tech Support had me download and re-install the Mac TB3 driver. My iMac Pro rebooted, and BOOM! It’s there! Thanks John D!!
If anyone else has this issue, you can find the drivers here (https://support.g-technology.com/downloads.aspx?lang=en) under Software By Product - > G-SPEED Software Utility - > MAC TB3 Driver.