Time Machine (TM) is Apple’s slick back-up facility. In the usual case, one will back-up (BU) to an external attached hard drive somewhat larger in capacity than the data to be backed-up.
To get the backup ball rolling, simply visit the TM preference pane, select the BU drive to use, and turn on TM. Incremental backups occur automatically, hourly for the current 24 hours, then weekly for the current month, then monthly. When the number of backups over time exceeds the available storage, older hourly and daily backups are pruned automatically. Ultimately, a disk full notification may result.
One can tailor the data to be backed-up by excluding folders. Else it all gets backed-up. TM will automatically exclude attached volumes including the BU volume. These can be removed from the exclusion list if they are to be part of a combined backup.
One restores any given file by entering TM, selecting the back-up time of the file status one wants to retrieve, then selecting the restore option. An entire backed-up drive can be restored through the Apple Migration Assistant utility (or via Setup Assistant if initializing a new Mac). Point the utility at a backup file and restored data is written to the new disk.
The Backup File
Apple uses a Sparse Disk Image Bundle file for backups because its structure supports incremental builds as add-on file ‘bands’. The file is named by TM for the computer on which TM is running, e.g. Big Mac.sparsebundle.
Back-up Multiple Mac Clients to a Network Server
Apple sells the pricey Time Capsule, a combined wireless base station and large capacity network hard drive, as a network backup appliance. But what if you have a network server already on your wired network and want to do the same trick to an internal drive on that server?
Apple discourages that; they’d rather sell you a Time Capsule. They do support this if your network has an Xserve running OS X Server. The server versions of OS X are able to bless their internal drives as Time Capsule destinations; client versions of OS X, not so much. So we need to roll our own.
There are two steps to creating a Time Capsule capability across a network of client OS X machines. First, you need to make the target backup drive visible to all the clients. Second, you need to create an initial Sparse Disk Image Bundle file for each client on this network drive. This will enable managing contention between competing TM backups without necessitating partition of the BU drive (partitioning is so old school).
Enable Visibility of Unsupported Network Drive on Client Mac OS X
Apple has for now left a back door switch available to enable TM visibility of ‘unsupported’ network drives. By default, this switch is set to off. So let’s turn it on:
defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes 1
Then the usual AFP sharing mechanisms apply. On the server, ensure File Sharing is on, and add the BU volume to the list of served items. On each client, log-in to the server and select the BU volume. Now the BU volume should appear on the client desktop and in TM’s list of available backup drives.
Prepare BU Drive for Time Machine Backups
On each client, open Disk Utility and select File:New to create a sparse disk image bundle of size X on drive BU (X should be about twice the amount of data to be backed up to this image). Name the file <computer name>_<computer MAC address>. The MAC address is found in the Advanced option of the Network preference pane, under Hardware or Ethernet tab. Omit the colons from the name. Now TM can find its initial backup image file for this computer. The file gets automatically renamed after the first back-up.
Note that the command hdiutil can also create this initial sparse disk image bundle. Further, if the initial file ever runs out of room, one can use hdiutil to increase the file size to permit further backup operations.
It is reported that the server need not be a Mac for this all to work, but I have only tried it with Macs.