So to report my experiences:

Brand new drive with ext4 filesystem on it. Intention is to move everything from 4 other drives and this replaces them all (maybe keep the biggest one which is 3TB)

I managed to get to about 170GB copied with mc running about 80-90 MB/s before it really fell away. I stopped it for other reasons and went with rsync to move on the fly so I can stop and restart it. Rsync is running around 20 MB/s now and seems to have levelled out there.

Edit: It appears Rsync is particularly slow and looks CPU bound doing disk to disk copies. Back to using mc I'm seeing 40-50 MB/s speeds.

I had already added it to MythTV's recording pool which it used straight away since it was empty of course. This occurred whilst copying. I did see a lot of "took a long time" (to write) messages but it doesn't appear to have lost any recorded data when writing.