Does a new computer require a new Time Machine drive? If I buy a new iMac to replace an old iMac do I need a new or different HD for Time Machine?
No - you don't need to get a new drive, but it's always nice to consider getting a new drive for reliability reasons. At some point, retire the old drive to the shelf or delete it and repurpose it. The backup should ideally be more reliable than the source in terms of longevity. Even an unreliable drive can be useful though since it will protect you from user errors and is unlikely to fail at the same time as the primary source if you are primarily concerned about having two copies of the current data.



One Time Machine drive can serve to hold files from many computers. Similarly, one computer can back up to multiple Time Machine drives.

In the case where you migrate data from one Mac to another, the system will usually detect this and offer to "migrate" the one time series of backups so that you get two benefits:


You don't lose the space savings where each file is only saved when it changes - saving space when a dozen backup intervals contain the same file version.
You don't lose history and have to wait for all the files to back up.