...
3) Each external HD may only have 1 computer backing up to it via Time Machine. Do not try to save more than one Time Machine backup per each external storage HD. Though a computer may be using Time Machine to back up onto more than one external HD, each backup external HD used for Time Machine may only have 1 computer that is backing up to it. Trying to backup more than 1 computer to any single external HD can cause the problem described in 2.
4) Recommendations and drives to look for. Good external storage HDs for use as Time Machine include Western Digital Passports or Seagate Backup Plus Slim external HDs also in 1-4 GB sizes. Prices for these devices are usually in the $100 to $150 price range. Be sure to check user reviews to make sure the drive you are looking at is reliable and user approved.