Apple Software Restore Fundamentals

The ASR process is essentially a highly optimized volume duplication system. When restoring locally, the source can be a disk image prepared for deployment or any mounted Mac OS formatted volume, including volumes that reside on external drives, optical media, and nonprepared disk images. Thus, you could actually use the ASR restore mechanism to make perfect clones from one volume to another. The only requirement is that the ASR restore process must be able to unmount and copy the source volume. This requirement limits you to source volumes that are not currently being used as a startup disk.

The restore destination volume can be any nonoptical storage device mounted locally to the Mac running the restore process. This includes any nonsystem volumes on a partitioned hard drive, volumes on external drives, and even volumes that reside in read/write disk images. The only requirement is that the ASR process must be able to unmount and replace the destination volume contents. Again, this requirement is why you cannot restore to a volume that is currently being used as a startup disk.

The size of your restore destination is also a consideration. Obviously, the restore process works only if your destination volume is as large or larger than the source volume contents. If the destination volume is larger, the ASR process will simply leave the remaining volume space as is. The exception to this is when restoring at the device level. The ASR process can also restore or clone an entire storage device, including all partitions, to another device. This technique is rarely used because the ASR process will have to reformat the destination device with the exact same partition scheme as the source device. Thus, the destination device must always be as large or larger than the source device, and any extra space on the destination device will not be formatted.

ASR uses two methods for duplicating a volume. The most commonly used method is the erase and copy. With this method the ASR process completely erases the destination volume and performs a very fast block-level copy. This method is often 10 times faster than the alternative, and it always results in the “cleanest” system restore.

However, if you need to retain the contents of the destination volume, you can perform a file copy. With this method the ASR process copies the items one at a time, replacing any items on the destination volume with the items from the source volume. It’s important to realize that the ASR process does not consider the age or version of individual items and will replace all items on the destination volume with a similar item from the source volume.

Finally, ASR can perform a verification of the restore. The verification process ensures that your restore or clone was fully completed without error to the destination. It does this by comparing the destination to the source. This verification doubles the amount of time it takes to complete the ASR restoration process, but it guarantees an error-free restoration or clone.