When you acquire an external hard drive, this is what you do before you use it for anything else. I assume you have the drive connected to your computer. For explanations of what is happening here, see “What has all this accomplished?” below.
CAUTION! The following procedures will ERASE EXISTING DATA from the external hard drive. So only do this on a new drive or on one whose data you wish to keep is already saved elsewhere; in this latter case, it is advisable to check that your data is still accessible from that alternative storage device before erasing it from the current one.
Open Disk Utility in the Utilities folder (which is itself inside your Applications folder).
In the Disk Utility window, select the topmost item representing your external disk, then click on the “Partition” tab and choose the number of partitions you want from the Partition Layout drop-down menu. [I’ve put 5 partitions, of equal size, on my 1 TB external hard disk.] Click on each partition in turn and name it. Click on the “Options” button and choose the GUID Partition Table (assuming the disk is only going to be used with Macintosh computers). When you’ve finished, and you are absolutely certain you will not miss ANYTHING on this disk which is now to be ERASED FOREVER, click on the “Apply” button. You’ve now partitioned your new external disk.
Going back to that left-hand pane of the Disk Utility window, now select the startup disk on your internal hard drive: this is the second item in the list, not the topmost. This time, instead of clicking on the “Partition” tab, click on the “Restore” tab. Your startup disk should appear in the “Source” field. (If it doesn’t, drag it from the left pane of the window to the “Source” field in the right pane of the window.) Choose one of the partitions you just made on your external disk and drag it to the “Destination” field. Click on the “Restore” button and agree to erasing that partition if asked. (If, however, you see an error message saying you cannot use your startup disk as the source and suggesting you use a recovery disk instead, Restart the computer while holding down the Option key. You will eventually be presented with a range of options for the startup device. Choose a Recovery disk and then select the option to open Disk Utility, after which you should be able to repeat the preceding parts of this Step with success.) Your startup disk will now be copied to the partition you chose on the external disk. How long it takes depends on various characteristics of your computer and external hard drive; for example, it took me a bit over half an hour to transfer some 60 GB of data from a MacBook Pro with an SSD internal drive to a LaCie external hard drive via Thunderbolt. When the job is finished, you can Quit Disk Utility. If you had accessed Disk Utility via a Recovery disk, choose Restart from the Apple menu in the top left corner of your screen and immediately press the Option key, continuing to hold it down until you are presented with the range of options for the startup device: this time, choose your usual startup disk.
Open Time Machine Preferences, either via System Preferences or via the counterclockwise arrow icon in the main menu bar at the top of your screen. Use your password to unlock Time Machine, click on the “Select Disk…” button and choose another partition (not the one you just copied your startup disk to) on your external drive to accommodate Time Machine backups. This is a good time, too, to click on the “Options” button to nominate files, folders or disks you wish to exclude from Time Machine backups. If the Time Machine switch hasn’t moved to “ON”, slide it so it does. Within a couple of minutes, it should start backing up. Wait until it finishes: once again, this could take some time.
On ejecting any partition on your external drive, you will be asked whether you want to eject them all. You do. Then you can disconnect your external drive from your computer.
Quite a bit, actually!
Each partition represents a single “volume”. Other examples of volumes in this sense are your computer’s internal drive, a USB storage “stick” and a recordable DVD. Your partitioned external hard disk is really the equivalent of multiple (smaller) hard disks. This explains the appearance of the names of all the partitions in a Finder window Sidebar or on the Desktop (depending on how you’ve set your Finder Preferences).
Partitioning the external drive reduces it to more manageable chunks of storage. Drives with lots of storage space are all very well, but doing anything on the whole thing at once can take an age. This becomes more apparent when things go wrong and you can’t afford to waste time unnecessarily.
Each partition can run a different operating system if you wish.
Time Machine is the easiest way to backup a Mac. It runs automatically in the background and just waits until you connect the nominated Time Machine backup disk (ie, the particular partition on your external drive), at which time it takes the opportunity to transfer its accumulated backups to that disk. Unfortunately, Time Machine doesn’t create bootable backups: you have to be able to get into your computer by some other means in order to use Time Machine for data restore purposes. Hence…
Step 3 above created a bootable disk on your external hard drive. A bootable disk contains an operating system that is capable of running your computer directly from startup. It can contain applications, documents and other files as well, but the point is that the disk doesn’t require an operating system other than its own in order for it to function. If your computer’s internal drive fails to boot, connect your external drive, and start up the computer while holding down the Option key. You will eventually be presented with a range of options for the startup device: choose the volume (partition) you used in Step 3. If it was only the internal drive that was at fault, the computer should start up and look just as though you had started it from a properly functioning internal drive. You don’t have to worry about re-installing software or anything. It will just work! At leisure, you can restore from Time Machine any data acccumulated since you created the bootable disk.
Ideally, you would have two external drives (stored in different locations), each with a bootable startup image and a Time Machine backup, in case of simultaneous failure of your internal drive and one of your external backup drives.
Time Machine does incremental backups, so subsequent transfers to your external storage device will take considerably less time (typically of the order of 5 minutes) than it took to create the initial backup. The bootable disk backup technique described above, on the other hand, will generally take longer on each occasion, as it starts afresh each time and the amount of data to be transferred is likely to be more, not less. There is nothing to stop you transferring copies of your data to the bootable disk without having to recreate the disk (eg, via AppleScript, Save or drag-and-drop), but periodic recreation of the bootable disk ensures you have an up-to-date version of the operating system, complete with Preferences, ready to go at a moment’s notice.
So you might use Time Machine every day, say, and re-create a bootable image of your internal drive whenever the mood moves you — maybe once a month or so.
Of course, for quick-and-dirty backup of a laptop on the move, it’s hard to beat dragging and dropping of files and folders onto a small but capacious USB (“stick”) drive — which is where my Two Finder Windows tip comes in handy.