Transfer external hard drive Time Machine data to a replacement
Have a look at: Time Machine: How to transfer backups from a current backup drive to a new backup drive

Synopsis:

In Disk Utility, select the new drive's icon to make sure it has a GUID partition and is formatted as Mac OS Extended (Journaled).  If not make appropriate changes.

Set permissions on your new backup drive in Finder using Get Info and make sure "Ignore ownership on this volume" at the bottom of the "Sharing &amp; Permissions" section of the Get Info window is deselected (unchecked).

Temporarily turn Time Machine off.

Copy your backup data from your original drive to your new drive. Drag the folder "Backups.backupdb" from the original backup drive to the top level of the new backup drive.

Set Time Machine to use your new drive.