So if I add a different backup disk from the one that is already selected will it wipe out all the backups records in TM and start fresh with the first backup on the new HD?
Just want to make sure I am starting from scratch when I add the new HD.
When you Add the new disk to TM it will start to use the new disk and it will have to do a full backup to the new disk which will take awhile.
TM will not delete all backups form the old TM disk. If you do not want to use the old TM disk then Remove is from the TM drive list. If you do not remove the old drive from the TM list, then TM will will continue to backup to the old drive (if it is attached to your Mac) and the new TM drive. If the old TM drive is not mounted, then eventually TM will start nagging you it has been unable backup to the old drive in x number of days if you do not remove it from the TM drive list.
If you definitely do not want to save the TM backups on the old drive then you can erase it using Disk Utility and repurpose it for some other task. Though, I would recommend not erasing the old drive for awhile until you are sure the new TM drive is working properly.