look in your system preferences.

Unfortunately, what you are trying to do is not supported by Apple, so you'll have to keep an open mind about all of this if you want to try it anyway.

It does seem to work for some users, it works for some other users some of the time, and some users have no success at all with this. I've tried this maybe 10-12 times over the past few years, and I remain in the "no success" category.

First, you need to make sure that the drive has been formatted in Mac OS Extended (Journaled)

Second, you probably will need to use a powered USB hub even though your drive is powered

Then you have to set up Time Machine as follows:

Open System Preferences (gear icon on the dock)

Turn "ON" Time Machine

Click Select Disk

If your disk appears (it may not....remember this is not supported) click on it to highlight it

Click Use for Backup

if the backup is successful, please keep in mind that most problems tend to surface a few weeks to even a few months after you start the backups. They become corrupted and you usually have to erase the drive and start over again.

So, you would probably want to set up a secondary back up plan as well to keep your important data safe.

but would it also be possible to simply plug the hard drive into one of the computers and use it as a shared drive on my network?

If you are asking about Time Machine backups here, the answer would be no. You would need to connect a hard drive directly to each Mac if you want to use Time Machine to backup the computer.

A Time Capsule would be able to back up all the Macs as a shared drive using TIme Machine. You could connect the Time Capsule to the AirPort Extreme using an Ethernet cable for that purpose.

If you simply want other computers to be able to access regular files on the hard drive, you could connect a hard drive to the AirPort Extreme for that purpose.....but not for Time Machine backups.

I realize this is an old post but the answers here are WRONG and this shows at the top of Google search results.

Apple does, in fact, support Time Machine backups to any USB hard drive connected to an Airport Extreme. It's the reason the Airport Extreme has a USB port and I've been using it as such for years across several generations of Airport Extreme. There's simply an additional step that is being missed here: you need to connect to the shared drive from your computer. Just plugging it into the Airport isn't good enough. Your computer doesn't know about it - you have to actually connect to the shared network drive from your computer before you can use it for backup.

The steps (from the beginning)...

1. Connect the hard drive to your Mac and format the drive as "Mac OS Extended (Journaled)".
2. Connect the hard drive to your Airport Extreme router.
3. Using Airport Utility, confirm that the drive appears in the disks window (greyed out is fine).
4. Select whatever secure method you like. If you choose "Device Password" you will use your Airport Extreme's admin login to connect.
5. Open Finder and select your Airport Extreme under "Shared" (it's the same place any shared computers/folders would show up)
6. Click "Connect" in the Finder window to login to your Airport Extreme's share partition.
7. Now the shared USB drive will be available in your Time Machine Preferences "Select Disk" window.

Apple does, in fact, support Time Machine backups to any USB hard drive connected to an Airport Extreme

No, they don't.  If the answers were wrong, the forum moderators would have edited this thread long ago. Check with Apple Support if you need confirmation. Or read this Apple Support document carefully:

Backup disks you can use with Time Machine - Apple Support

Some users actually have been able to make Time Machine backups work with the earlier versions of the AirPort Extreme for a time. Most have not. We've beat this to death on the forums.

Even if you can get it to work for awhile, you will receive no help from Apple in the event of problems.......unless.....you are using the newer 802.11ac "tower" version of the  AirPort Extreme.

If you are using an earlier 802.11n version of the AirPort Extreme, expect to have corruption problems with your backups.  The only question is "when"......not if.

Good luck!

Excellent advice Veraxus - worked like a charm. In fact, once I accessed my Airport Extreme via "Shared", I simply dragged the hard disk volume directly to the "connect" and boom - on it goes.

Be prepared to see this message if you are going to try an unsupported method.  The message will appear......it might be a few days, a few weeks, a few months.....but it will appear. When it does, it's too late. Your backups are already corrupted.

Feeling lucky? Post back in a few months to let us know how things are working.  Good luck.

I have had this problem too. Are you suggesting that this text from Apple support is wrong and that Time mAchine is not usable with an Airport Extreme a/c router ?

"

Backup disks you can use with Time Machine

You can use Time Machine with a drive connected to your Mac, a drive built into or connected to a Time Capsule, or a supported network volume.

Time Machine can back up the data on your Mac to these backup disks:

• An external USB, Thunderbolt, or FireWire drive connected to your Mac
• An AirPort Time Capsule's built-in drive (any model)
• An external USB drive connected to an AirPort Time Capsule (any model) or AirPort Extreme (802.11ac model only)
• Network volumes connected using Apple File Protocol (AFP)

Are you suggesting that this text from Apple support is wrong and that Time mAchine is not usable with an Airport Extreme a/c router ?

No, the Apple statement is informing you that they DO support Time Machine backups to an external USB hard drive connected to an 802.11ac-version of the AirPort Extreme base station.

They DO NOT support the same backups to an 802.11n version of the AirPort Extreme.

Sorry, but you did not read my posts above carefully.  I said previously that......

Even if you can get it to work for awhile, you will receive no help from Apple in the event of problems.......unless.....you are using the newer 802.11ac "tower" version of the  AirPort Extreme.

Please note that the original question was posted in February of 2012.  The only AirPort Extreme versions that were available at the time were the  802.11n "flat or square" shaped models.  Time Machine backups to a USB drive attached to any of these 802.11n versions were not officially supported by Apple.

Despite the fact that Apple did not officially support Time Machine backups with a USB drive with the older 802.11n versions of the AirPort Extreme, some users were actually able to make this work. But most users...including me.... had difficulties right out of the gate or within a few weeks to months after they set up the backups.

The "tower" version of the AirPort Extreme was introduced in June of 2013, which was some time after the original question was posted. So, my answer at the time of the original post was correct.

Apple does officially support Time Machine backups to the 802.11ac "tower" version of the AirPort Extreme, if you want to try this, but a note of caution if you do......and that would be that the port on the 802.11ac is USB 2.0, which is much slower than the standard USB 3.0 ports in use on most products today. If you intend to back up over wireless, then things will be even slower that way.

Thanks.

Then i will just have to wait and see if the "start new backup" option reappears. I did a hardware reset on my AiportExtreme (a/c modell) with update 7.7.7 and a discformatting

Please do not wait to see if "Start New Backup" appears if you value the data on your backups. Once that message appears, it is too late.

If you want to improve your chances of success and reliability with Time Machine, connect another USB drive directly to your Mac and set up backups that way, in addition to using the drive connected to the AirPort Extreme.

You will have much fewer Time Machine problems if you can connect your backup drive directly to the Mac backing up. If that is not convenient for day to day use, then connect the drive maybe once a week and run a backup.  That way, when....not if...you have a corruption issue again with the drive at the AirPort Extreme.....you at least have your backup data safe on another drive.

Apple might officially support Time Machine backups to a drive connected to the 802.11ac AirPort Extreme, but they still won't do anything if you lose your data due to a problem.

I had similar issues to the OP.  I followed Veraxus's steps above and it worked instantaneously. 

My 5th gen 802.11n Airport Extreme is joining a Nighthawk network, it's only purpose to to bring a Time Machine USB-powered drive onto the network.