Not quite sure what you're trying to do using the "Time Machine" program to back up an existing Time Machine Backup, but there is a very good little program called SuperDuper which will clone a hard drive. This would let you create an exact copy of your 2TB Lacie drive on the new 8TB drive. Be aware though that SuperDuper will erase anything else on the destination drive. You will only have a clone of the original drive.
Microsoft Surface Book 2 15in review