Best practices for proper care and feeding of your external storage HD for Apple's Time Machine backup
1) As a general best practice all backup drives should be replaced with new drives when the machine is replaced. The average life span of a backup HD is 3-4 years. Once a machine is retired, its Time Machine backup drive should also be immediately retired and a date written on the drive to indicate the last time it was used.
2) Users should never save anything ELSE to a Time Machine backup drive. We know it is tempting to save a movie or photos on what looks like a mostly empty drive but please do not use the Time Machine storage HD for anything else. The few times we have seen Time Machine external drive failures users were copying other files on the drive unrelated to the Time Machine backup. Storing other files unrelated to Time Machine on a Time Machine drive can cause fatal errors to occur in Time Machine that permanently breaks the backup.
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. If you have a lot of files, make sure you get a large enough backup drive. As a rule of thumb, check to see how much storage space you're using on your machine's HD and then multiply by 4. Round up the closest size of 1TB, 2TB, 4TB, or 5TB. Be sure to check user reviews to make sure the drive you are considering for purchase is reliable and user reviewed with a very high ratings. On Amazon, a 4.5 star rating is a good starting point.