But be aware that: You need to have the disk connected for back ups to happen (fine on a desktop, but not guaranteed with a laptop), it’s slow to restore from if you replace a failed internal hard disk (you can’t boot from it), and it offers no protection against theft or local disasters such as fire.
Time Machine to a disk connected to your network
Alternatively, you can use Time Machine to back up to a disk that is connected directly to your network rather than to a specific Mac. This means it’s available to all the computers on your network so you can have them backing up centrally, and best of all, they back up completely automatically over the network every hour (either via Wi-Fi or ethernet, depending on how they connect to it). This is great for laptops especially: now you don’t have to remember to connect your backup disk; it just does it automatically whenever the Mac is awake.
Most people will think of Apple’s Time Capsule for this method—a network router with a built-in hard disk—and indeed it’s the simplest option, but you don’t have to go with that. Plugging a hard disk into an AirPort Extreme’s USB port will make it available on the network for Time Machine, and lots of other Network-Attached Storage (NAS) devices from companies other than Apple also support Time Machine backup. Indeed, they can offer other features besides; see “Fire- and waterproof disks,” below.
Good because: All the advantages of Time Machine, but more convenient, especially for laptop users, since backups happen automatically over your home network.
But be aware that: Backups are a little slower (or indeed can be muc slower, depending on the speed of your network or the bandwidth of the method by which your Macs connect to it) and restoring is even slower still. You have to be connected to your home network. A little extra complexity. And no protection against theft or other local disasters.
Cloning your disk to another connected to your Mac
Apps such as SuperDuper! and Carbon Copy Cloner let you mirror the contents of your internal disk to an external drive, and can set schedules so that this happens, say, daily at 3 a.m. so that it’s not getting in the way of your work.
If your internal disk fails, you can just boot from the external clone and continue as if nothing had changed. Obviously, you don’t want to rely on this solely, or be doing it for long; it might be slow, and it means you’re a level of redundancy down, so that if the external clone fails, you’re in big trouble. But you can’t beat it as a way to keep you running while you get the internal fixed.
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