If you re-format the drive in the middle of a Timemachine back up

[levmc]

Does that abort the back up? (That's what I did and it seems like the backup was aborted because I don't see the time machine window that shows percentage complete so far)
Also how long might it take to backup 1TB?

 

[0128672]

Yes, it would abort any running processes. How long it would take to back up 1TB depends on many factors, and no one could say with certainty. The first backup is full and subsequent backups are incremental, so would not take as long.

 

[glenthompson]

Too many factors to figure out the time - usb speed, disk speed, interface components, etc. with usb3 it should be able to complete overnight. Initial backups over WiFi to a Time Capsule can take a very long time.