Mac – How to back up two internal hard drives using Time Machine

Yes - when you set the Time Machine to back up to the Time Capsule - you will include the internal drive by default. I would let a couple of backups complete, and then go to the settings for time machine under options and make sure the external drive is not excluded. Kick off a a backup with the external drive connected to make sure it works.

If the drive isn't connected when a back up event arrives, the internal will get backed up - but the software will remember and resume backing up the external any time it is connected and a back up happens. You don't need to always have the external drive connected for this to work, you just lose some chances to back it up when it's not connected.

Yes, Time Machine will backup until it fills the backup disk and then it will start overwriting old backups to make way for new ones. However, if the volume of data you've told Time Machine to backup exceeds the size of the disk Time Machine is using to save the backups on, it will complain that the disk is too small. If you have a Time Machine disk which is smaller than the combined size of the hard disks used by your Mac, it's likely you'll need to manually intervene at some stage due to this issue unless you're very diligent about ensuring that you don't ask it to backup too much data.