I use Time Machine with two backup hard discs.
Does Time Machine create an identical copy of the data on both disks, or does it back up more fine-grained snapshots to different disks, so that I have more choices regarding the timestamp to restore to?
Let's say, with only one disk, Time Machine backs up hourly snapshots — 4:00, 5:00, 6:00, etc. Does it back up snapshots taken at 4:30, 5:30, 6:30 to the second disk?
Best Answer
Time Machine will continue to perform hourly backups, rotating through your backup drives. Each drive will contain a full, independent backup of your source drive.
This provides redundancy for your backup system, if that is what you seek. In case one of your backup drives fails, you still have another. This can prove particularly useful if you keep one of those backups offsite (backing over the Internet will require special setup, however, and can prove agonizingly slow).
The ability to use multiple backup drives was introduced starting with OS X 10.8 (Mountain Lion).