Macs make automated backup childishly easy: simply plug in an external hard drive and OS X will ask whether you want to use it as a Time Machine disk. Say yes, and you’ll then get fully-automatic, hourly, versioned backups without doing anything further.

Unplug it to take your MacBook out & about, and it will catch up as soon as you return and plug it back in. Even easier, get a Time Capsule, and those backups take place over wifi, so you don’t even have to connect a drive.

But I’m a belt-and-braces chap. I like multiple backups, and I like one of those backups to be off-site. That way, if the house burns down, or a burglar takes both my Macs and my backup drives, I still have access to my data. Which is where online backup services come into play. Think of them as your backup of last resort.

iCloud, covered in my cloud storage roundup last week, already backs up quite a lot of your data – but nothing like all of it. The services covered here are ones that backup either your entire Mac, or a large proportion of it …

So, how do online backup services work? You download an app to your Mac that creates a backup of your machine (caveat in a moment) on a remote server. That initial backup will take quite some time: days or weeks. The reason is that while you may have a lightning-fast broadband connection, the speed quoted is for downloads. Upload speeds are typically 10-20 times slower.

But once the backup finally completes, the backup app sits quietly in the background keeping it updated. Every time you create, change or delete a file, the change is mirrored on the backup server. Since it’s online, it does this anytime you have an Internet connection, which means it works as well when you’re in a hotel on the far side of the world as it does when you’re at home.

I mentioned a caveat: most services do not backup your entire drive, only the data files. So you’ll get back your photos, music and documents, but usually not your apps, preferences, settings and so on. There are a few services that give you the option to backup everything, and I’ll mention those as we get to them. Note too that many services don’t support multiple drives: if you have more than one drive in your Mac, only the main one gets backed-up. You can see below the exceptions.

As with cloud storage, there are a huge number of competing services, and my view is that you are safest with the major players. Small startups come and go all the time; not such a problem if you bought a $10 accessory from them a year ago, rather a bigger problem if your backup disappears along with the company. I’m thus again focusing on the best-known companies: Backblaze, Bitcasa, Carbonite, CrashPlan and Mozy.

Backblaze is one of the best-known services out there. It focuses purely on backing-up a single machine: there are no file-sync features, and no public sharing of files. It backups your entire Mac, the software is easy to use and upload speed is decent. If the worst does happen, and you need to restore a trashed, lost or stolen Mac, there is the (paid) option of having your backup sent to you on an external drive rather than having to wait for everything to download.

Cost: $50/year Capacity: Unlimited Supports multiple drives: Yes

Update (24th Oct 2014): Bitcasa has now ended its unlimited backup service.

Some readers wondered why I didn’t include Bitcasa in my cloud storage roundup. The reason is because although it primarily presents itself as a cloud storage service, it offers a mirroring option that makes it ideal for backup, hence including it here instead. Just right-click on the top-level folder you want to backup and select the mirror to Bitcasa option.

Bitcasa is one of the most secure service available, offering AES-256 bit encryption. It is also probably the most trustworthy when it says you can store unlimited data. ‘Unlimited’ is generally shorthand for ‘see small-print for details’, but what Bitcasa does is rather clever.  The company estimates that most people have no more than 25GB of unique content on their computers, the rest – music, movies, etc – being content held in common with other people. For that data, Bitcasa stores only one copy of the file, with the rest of our backups containing a pointer to it. That means faster initial uploads. Neat.

Cost: $99/year Capacity: Unlimited Supports multiple drives: Yes

I’m including Carbonite because it’s one of the best-known services, but it’s unfortunately not one I would recommend. It’s predominantly a Windows service, with limited options for Mac users (no option for multiple drives even at extra cost, for example). Last time I used it – admittedly a couple of years ago – the Mac app also started eating CPU cycles for breakfast.

Although the service claims to offer unlimited capacity, it throttles upload bandwidth after the first 35GB, so if you have a lot of data, that initial upload takes a very long time. On the plus side, there is a $229 option for 250GB across unlimited computers that may be a good deal for some families and small businesses.

Cost: $60/year Capacity: Unlimited (but see above) Supports multiple drives: No

I’ve been using CrashPlan myself for a couple of years. One of the key reasons for choosing it is that it genuinely allows you to backup your entire machine, system files and applications included. Ok, you’ll probably never want to recover apps that way, but I can be confident all my settings and preferences are available in my cloud backup as well as my local ones.

I also serve as a good test of that ‘unlimited’ claim: with two 1TB drives in my Mac, totalling around 1.4TB of data, the initial backup took a few weeks, but it all got there and with no visible throttling. Like most services, it doesn’t upload everything as standard, but unlike most you can tick boxes to make it do so. A menubar mini-app lets you track backup status, while you open up the main app to take a more detailed look or change settings. CrashPlan uses CPU cycles intelligently, using more of them when your Mac is idle and fewer when it’s in use. You can also pause upload if you want to maximise your upload bandwidth for something else.

Cost: $60/year Capacity: Unlimited Supports multiple drives: Yes

Mozy was one of the earliest online backup services, but has now fallen somewhat behind the curve, no longer offering unlimited capacity and with no support for multiple drives. Instead, you have to choose between 50GB on one computer for $66/year ($5.99/month with one month free for annual payment) or 125GB on up to three computers for $110/year. The website looks a bit 2005 too!

However, it can be a good deal if you want to protect more than one Mac and don’t have more than 125GB of data in total. There are also options for adding additional capacity and extra computers. Mozy also supports file-syncing between computers through its public beta of Mozy Stash (a free upgrade for any paid account), so it’s worth looking at for multiple Mac owners.

Cost: $66/year Capacity: 50GB (or 125GB for $110) Supports multiple drives: No

There are of course many more services out there. Of the big ones covered here, Backblaze offers the best deal, at $50/year for unlimited capacity and with support for multiple drives. At $60/year, CrashPlan costs a little more, but has served me well. Bitcasa will probably give you the quickest initial upload and highest security, if you’re willing to pay a little more for it ($99).