Helping people with computers... one answer at a time.

It's possible to place the backups for multiple machines onto a single external hard drive; I'll look at the issues and why you might not want to do this.

I'm new at this backing up stuff. I've lost some information over the last few years, but I don't put my life on my computers. I'm old fashioned and keep PAPER copies, DVD, and CD Hard copies. Anyway, I was wondering if you can mix your formats on say a one-terabyte "My Book Essentials" external hard drive. I want to put two computers and one lap top on it and ALL of them have less than 50 GB each, so a total of approximately 150 GBs. Now my problem is, one is a Windows 7 SP-1, XP Pro SP3, And a MACBook Pro SnowLep. current version. What would be the best division of the one-terabyte hard drive and can I format the division for the MAC by itself ? I just want to clone all of my HDs as they are now. Is this even possible? Like Clone, Backup, Bootdisk needed, which program do I need to do these things with? I guess I want to clone my HDs in case they crash or they get infected by a bad virus and I want to just be able to put a clean operating system back in its place with all my programs working as they are today. I have free Avast on the Win. 7 machine and free Avira on the XP and free Sophos on the MacBook plus a ton of anti-spyware, like SuperAntiSpyware Pro, CCleaner, etc.

The short answer is that it's quite possible to place more than one machine's backup on a single external drive. I'll admit up front that the Mac's going to throw me a bit of a curve because I'm not that Mac-savvy. Perhaps some of my readers will chime in with some suggestions.

I'll review the tools, terminology, and, perhaps as important, the habits that you'll need to adopt for this to be successful.


While "clone" is somewhat correct, what I believe that you really want is more properly called a "system image" backup, along with periodic incremental backups to save your most recent work.

“Creating a periodic system image, say once a month, and then augmenting that with a daily incremental backup gives you the best of both worlds...”

A "clone" most commonly refers to a sector-by-sector copy of the hard drive - copying everything, even the empty space on the drive. (There's no formal definition of the term, and some may use it to mean a copy of only the data.) A clone is a complete image, and not something that can be updated. If your data changes, as it will as you use your computer, then to back it up, you would need to create an entirely new clone of the drive.

That's both time consuming and somewhat wasteful.

A "system image", as I and many others use the term, is a copy of all the data on your hard drive including the operating system, programs, data and whatever else that might be there when the backup is taken. Most programs that allow you to take a system image backup also provide for the ability to take what are called "incremental" backups; copying only those files that have changed since the immediately previous backup.

Creating a periodic system image, say once a month, and then augmenting that with a daily incremental backups gets you the best of both worlds: a complete backup of everything on your system that uses both your time and your disk space more efficiently.

External Hard Disk


For a Windows-only scenario, I'd advise simply creating a single partition on an external hard disk and, in that partition, creating a folder for each machine that you plan to back up. Backups are simply stored as files, and thus, you can name, organize, and place them in whatever way makes sense to you.

You'd then configure the backup software on each machine to place the backups in that machine's associated folder. That'll keep all the backups organized by machine, while keeping them on the single external hard drive.

Mac's Time Machine - the excellent backup software included with Mac OSX - throws a wrinkle into this: I believe that Time Machine requires complete ownership of the backup drive, and uses a a Mac-specific (if not Time Machine-specific) hard disk format.

I believe, and the Mac folks will have to help me out here, that you could partition that external hard drive and dedicate one of the partitions for Time Machine's use and the other partition for the rest of your Windows-based machines.


Time Machine is the standard on Macs. I really don't have reason to recommend anything else.

For Windows-based machines (XP, Vista and 7), I use Macrium Reflect.

Naturally, there are other programs that can do the job as well, and it's worth it to do the research to find out which ones. Macrium has worked well for me. The key is that whatever software that you choose should be able to:

  • Take a complete system image of the entire computer, not just your data.

  • Support the ability to perform periodic incremental backups.

  • Include the ability to do what's called a "bare metal restore".

That last point is worth discussing in a little more detail.

Bare Metal Restore

Imagine that your computer's hard drive dies, and is completely unrecoverable. Toast. All you can do is replace it.

You're not worried, though, because you've been carefully backing up every day as I've described above. You have a full-system image and a few incremental backups taken since that are ready to go on your external hard drive.

The computer won't boot because the replacement hard drive is empty. Yet, you need to run your backup software in order to restore your backups to the drive.

The solution goes by many names, but I call it "bootable rescue media".

The software that you use to back up should include or allow you to create a bootable CD - your bootable rescue media. You make this when things are working and save it for the day you need it. You can also make it using a different computer. When it comes time to restore your backups to your new, empty hard drive, you boot from the rescue media, a version of your backup software runs, and you use that to then restore the backup from your external drive.

Whatever solution you use, make sure that it can do that or you won't be able to restore when you need it most.

Those Habits I Mentioned...

There's one problem with the specific scenario that you've described: you need to physically move the hard disk from machine to machine in order to enable each machine to backup to it.

Absolutely, positively something that can be done.

And it's also extremely easy to forget.

Yes, it's possible to attach the hard drive to a single computer and then share that hard disk so that other computers might use it. The problem is that these backups are typically large (especially that periodic system image) and networks are slow compared to direct connections. Again, it can be done, but networking can at times be problematic and possibly too slow to be practical, depending on your configuration.

And, again Mac folks can correct me if I'm wrong, but I believe Time Machine requires that the drive be directly connected to your Mac.

So, you need to develop a very good habit of moving the external drive from machine to machine to ensure that backups are happening on all.


Drives are relatively cheap.

Get one for each machine and avoid several issues all at once.

Article C4782 - April 1, 2011 « »

Leo Leo A. Notenboom has been playing with computers since he was required to take a programming class in 1976. An 18 year career as a programmer at Microsoft soon followed. After "retiring" in 2001, Leo started Ask Leo! in 2003 as a place for answers to common computer and technical questions. More about Leo.

Not what you needed?

April 5, 2011 8:47 AM

Hi - thanks for this useful article.
Like you, I'm not Mac-savvy, but my wife is a Mac enthusiast and insisted on getting a Macbook with Snow Leopard (OS X 10.6) which, as you say, includes Time Machine.

I have a huge spare external USB hard drive so I first divided it into 2 non-equal partitions using Windows, Partition magic or whatever software, and then connected the drive to the Mac. After a bit of thought, Mac offered to format some or all of the partitions it found. I told it which one to use, and Time Machine has been running happily ever since.

In this case with 2 Windows PCs and one Mac, I would still use only two partitions. Use partition A for all Windows backups and partition B for Mac backups, and create one folder in partition A in which to store all backups for each PC (eg create folders PC1 & PC2).

You probably want partition A to be 70% of the hard drive and B to be 30%. The reason for having them different sizes is so you know for certain which is which as you don't want to re-format the wrong one by mistake :-) .

April 5, 2011 9:12 AM

The first thing you want to get when backing up a mac is a piece of software called CarbonCopyCloner. It will back up everything and if your mac is capable of booting from a USB drive it can boot from your backup.

Jonathan Johnson
April 6, 2011 7:46 AM


I am a both a pc and a mac user and I can shed some light about backing up.

Time machine (built into Snow Leopard) is a great choice because well, it is free (again built into snow leopard). You will want to partition your hard drive (which Snow Leopard will do) to the format called Mac OS Extended (journaled). That is the only format TM (Time machine from now own because I am lazy) will use.

As far as how to backup multiple computers at one I've networked my external hd to my router via usb. As long as your mac can see the external (it will show either in finder under shared or on the desktop), time machine is ready to go. Also, You can purchase a router from Apple called Time Capsule, which has a 1TB or 2TB drive built in (as well as a usb port to add additional HDs). I agree with the previous reader by saying keep your partitions to 2. One for your pcs and one for your mac. Be aware that Apple does recommend for time machine you use an external HD that is twice the size of your mac's hd, but TM does delete the oldest backup when it runs out of room on the partition or drive. Hope that helps

Royal T
April 12, 2011 11:00 AM

I have "stuff" from three dif 'puters on one external hardrive... I just created a "Folder" for each dif 'puter that I save/transfer to; in each "Folder" I create "Files"... same as when saving anything on my 'puter's harddrive.

I find no problems with using my 500GB Seagate External Harddrive this way.

But I DO NOT have a Mac!

Carlos R Coquet
April 29, 2011 11:43 AM

In a society where at any point in time millions of automobiles are polluting the air in drive thrus because people will not do something as simple as park and walk in to get something, backing up is another "casualty" of laziness. Backups that you leave connected to a computer only protect you against one thing: original hard disk failure. They do not protect you against fire, equipment theft, or viral attacks. Face it, the only true protection is to connect the backup device each time, do the backup, then disconnect the device and store it away from the data it copied. PERIOD !!!

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