If you upgrade to a new hard drive with a larger capacity, you may want to transfer your existing Time Machine backups to it before using it for regular backups. To keep your existing backups on your new, larger backup hard drive, follow these steps:
1) Connect your new hard drive.
2) In Disk Utility, make sure it has a GUID partition and is formatted as Mac OS Extended (Journaled). If needed, reformat the drive as Mac OS Extended with a GUID partition.
3) Select the new drive's icon on the desktop and choose Get Info from the File menu.
4) Make sure "Ignore ownership on this volume" is not enabled.
5) Open Time Machine preferences in System Preferences.
6) Slide the Time Machine switch to Off.
7) In Finder, double-click the current backup hard drive to open its Finder window.
8) Drag the folder "Backups.backupd" to the new hard drive.
9) Enter an administrator user name and password, then click OK to start the copying process. This may take some time to complete because all your backups will be copied..
10) In Time Machine System Preferences click "Select Disk…"
11) Select your new hard drive, then click "Use for Backup."
Now, on your new hard drive, you will have all of your existing backups from the previous backup hard drive plus have more room for more new backups.