Gappers wrote:
1. Can I back up to the TC and then use the external drive as, well, an external drive, for extending my internal storage?
For Time Machine use, NO, as TM does not span over more than one drive. You can use the second drive as an archive if attached to the USB port of Time Capsule or as network-attached storage device.
2. Then, is it possible to back up both the files on my internal HD and the files on the external HD connected to the TC to the HD in the TC? (space permitting)
You can do TM backups using TC as the destination, but the new external HDD needs to be attached
directly to your MacBook. TM does not back up a NAS (i.e. the USB HDD attached to your TC).
Another back up utility (not TM) may work with your desired setup (e.g. SuperDuper!).
Good luck and welcome to Apple Discussions!