On Tuesday I received two new 2TB hard drives for my backup server.
I hooked them up, and used LVM to put them in to a 2TB mirrored configuration.
Three days later (i.e. today) LVM finished syncronising these two empty hard drives.
W. T. Heck.
Once I’ve moved the existing data on to the new drives, I’ll have to do the same thing with the existing 1TB drives 
Yup, just going through the same sort of process here. How can it be so slow to synchronise an empty voume???