Adding 2nd HDD to Time Capsule - best practice?

Should I have copied the existing backups from the TC internal HDD to the new external HDD first, so that both backups are continuing from the same point, with the same 'history',

IMHO you are doing it the correct way by backing up afresh.

History is not that important after a few weeks and months... if the computer was stolen for example.. what you want to do is to restore the computer as is.. and what you were doing last year is really of little relevance.. Even if the Gen4 TC goes bad, the history is never likely to be lost from the Gen4 TC.. it simply needs the hard disk removed to get at it. But it is the current image of the computer that is still the most important.

We have found copying backups not hugely successful. Although in this case it would have been possible by doing an archive.. still if you asked the question of what you should do.. Myself and several others here would always recommend a new clean backup as being the most reliable method.

Would I have been better off connecting this new external HDD direct to the iMac, do the first backup and then connect it to the TC to continue it's alternate incremental backups? Again, is that even possible?

This is a good question.. EOP adapters can be very slow. Slower even than wireless AV200 are a fairly old standard and the actual speed could be down around 30Mbps.. it might surprise you.. and indeed should have been measured before you started.

The problem is Time Machine backs up differently to local cf network drives.. when you plug the USB drive into the TC it is then a network drive and the backup will not be recognised.

Pondini wrote up a method to get around that.

See Q18 here.

http://pondini.org/TM/FAQ.html

I have not tried it.. Bob Timmons who is regular poster here has and discovered it was unreliable. I think a backup is too important to fiddle with in this way.

However leaving the USB drive plugged into the Mac and keeping the backup local makes very good sense. It is much faster and more reliable than backups over the network.

If you really want to have the backup on the WD drive plugged into the TC, then I recommend you do it via ethernet.. remove the TC from the network if it is too far for a patch lead.. you can save the current configuration (export in the top menu of airport utility).. then plug it into the computer directly with a short patch lead.. and the USB drive into the TC.. the backup will be many times faster.. less than a full day (24hr) for 1TB if all goes well. Once completed you can then put the TC with the USB drive back into the network location.

Another question, if I may:

At the end of this I'll end up with two useable backup disks, one inside the TC and my external WD HDD connected to it via USB. When it comes to restoring from the external HDD, will that only ever work if it's still connected to the TC, i.e. I won't just be able to connect via USB to the iMac?

Good question.. the answer is that you can indeed use the USB drive plugged directly into the computer. Time Machine requires the backup to be placed in a sparsebundle remotely.. but it has no issue reading it locally.. It is simply that it is not required for local backups.

There is some logic or venn diagrams or something.. you cannot backup locally and read remotely.. but you can backup remotely and read locally.

(Although that is simplistic and I had to edit so you realise this is not 100% the Truth.. this is more practical truth.. if that makes sense).