I have two external 4TB disks, one for data and the other for TM backups. I recently moved from a MBP2010 (10.8.4) to a MBP2012 (10.10.5, fresh install).
On the backup drive, TM adds a root folder for the new machine, and that's fine. But it doesn't recognize the data disk, wanting to back it up from scratch.
There are many posts about TM behavior, but I haven't seen anything about this scenario. Yet there's a ton of people who use large external drives for data. Everyone has to wipe their backup and start anew when they migrate?
(PS I'm fully aware that the backup media is too small and if my 4TB data disk reaches closer to capacity I'll have to get a large backup drive.)