Reformatting of the entire drive only takes place if/when the external is physically mounted via a direct cable connection. Not sure, but if your multiple-Mac-using colleagues are backing up to the same drive, chances are they're using Wi-Fi rather than running around the office shuttling the backup disk back and forth. ...
My bad. They use multiple 1TB drives for multiple machines (ie, one drive per each machine) via direct cable connection, swapping them out daily to secure off-site storage.
