Time machine: moving a source external disk to a new computer 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.)
To make sure I understand - you have one external HD for data, plugged into your new computer, that was previously attached to your OLD computer - and you backed up your old computer + this drive to a second Time Machine drive?

Time Machine is not recognizing the old archive because the new machine is a fresh install. So it made a new backup. The data drive (which you state is being seen as a "new drive") would have been part of that old machine backup - Time Machine should recognize things on a per-computer, not per-drive basis. Unless you tell your new machine to inherit the old backup, the existing backup of your data disk will be ignored as well, and a new backup will be created with that disk as part of your current computer backup.

SSDs are great but the lack of cheap internal storage means more people have to use external disks for data. Cloud isn't a good alternative for everyone. Unfortunately the backup systems designed by Apple are not treating externals as separate backups - they are included as part of the internal mounted disk hierarchy of your computer.