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Using a networked drive for Time Machine backups (on a Mac)

You'll find similar information to this around the web, but I find it fiddly enough to piece together reliably, and I need it often enough, that I thought I'd blog about it. That way it at least gives me a single place to look. Maybe it will help others too. Much of the specifcs, especially the hdiutil command line and the ifconfig trick, I sourced from this thread in the ReadyNAS forums. Note that the advice is by no means specific to ReadyNAS drives (I have a Thecus NAS myself). Many thanks to btaroli in that thread for the insight.

Time Machine

Time Machine is Apple's easy-to-use backup system, baked into OS X (as of Leopard). Unfortunately it doesn't allow you to back-up to a networked drive out of the box. Enabling this ability is pretty easy. Early on there were some reliability issues - which were largely due to the fact that Time Machine created a disk image (more specifically, a sparse bundle) on the network drive, and this was prone to corruption if the network connection was disrupted during a backup. I don't know if all the issues here have been entirely resolved now, but it does seem more reliable. Apple's own Time Capsule, which has been specifically designed to work with Time Machine, uses this same method, so it is no longer an entirely unsupported technique.

Enabling Time Machine for network drives

So how do you enable backing up to network drives? Open a terminal window and paste the following in (then hit return, of course):
defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes 1
Mounted network drives will then show up in the list of destinations available for storing backups.

Getting a working disk image

Unfortunately this is not always enough. Often, after doing this, Time Machine will appear to start preparing a backup then fail with a cryptic error code. The error I have seen is:
Time Machine could not complete the backup.
The backup disk image "/Volumes/backups-1/Wall-E.sparsebundle" could not be created (error 45).
"Error 45"? What's that. If I try to create a sparse image myself in the same location I'm told, "the operation could not be completed". This is not much more helpful. If you google there are many references around to these errors - mostly in forums. Many of them are not terrible helpful, or require a lot of knowledge and/ or patience. I still don't really know what the problem is, although I suspect it's something to do with permissions and/ or attributes. Either way the solution generally seems to be to create the sparse image manually using a command called hdiutil. If you get this right then Time Machine will think it created it and just start using it. Simple eh? Well, it's not rocket science - but it does involve piecing a few things together. The name of the sparse bundle has to be something very specific which is made up from a few pieces of information unique to your set-up. I'll now take you through how to find those pieces of information.

Finding the Computer Name

We'll start with the easy one. The computer name. Specifically this is whatever the computer is named in the Sharing preferences. So open System Preferences, select "Sharing", and copy the name from the "Computer Name" section at the top.

Finding the MAC Address

This is the physical address of your network card (not your IP address, which is a logical address. Also the term "MAC" here is nothing to do with your Mac as a computer - it stands for Media Access Control address). Now you have to be careful here. Most macs these days have at least two network cards! You will probably have an ethernet port (for a network cable connection) as well as wifi. You may also have a USB based device, such as a mobile broadband device. Regardless of which one you use to connect to the network drive you'll be backing up to, the address we need is of the first network card (usually the ethernet port). If this seems a bit odd at first, consider the case where you usually connect over wifi, but to do an initial backup you connect by cable. If the backup name was dependant on the network connection used this wouldn't work. The address is only used to identify your computer. Anyway, it turns out there is an easy way to obtain this. Back in the terminal window, type the following:
ifconfig en0 | grep ether | awk '{print $2}' | sed 's/://g'
What's that doing? The short answer is "don't worry, it works". The slightly longer answer is that ifconfig dumps all the information it has about all it's ethernet ports. The first port is called en0, so the command ifconfig en0 dumps information about just that one. The pipe character, |, is the unix instruction for sending the output of one command to the input of the next. So we send the information from en0 to "grep ether", which filters out just the lines that have the word "ether" in them - which in this case happen to be where the MAC addresses are shown. To get that line into the form we need for our filename we pipe it to another command, awk, which just picks out the second part of the string, then finally to sed, which removes the colons. Phew. Like I said, it just works. Trust me.

Creating the sparsebundle

Now we have the information we need to create the name of the sparsebundle. Following is the instruction you need to issue to create it. Replace the <mac address> and <computer name> placeholders with the information we obtained above. You may need to change the size parameter (320g here) if you have a large drive to back up. The disk image doesn't take up that space to start with, but will grow up to the size you specify here, so use it to set an upper limit. Also you will be prompted to enter your admin password (sudo runs the command as SuperUser):
sudo hdiutil create -size 320g -type SPARSEBUNDLE -nospotlight -volname "Backup of <computer_name>" -fs "Case-sensitive Journaled HFS+" -verbose ~/Desktop/<computer_name>_<mac address>.sparsebundle
Note that this will create the sparsebundle on your desktop. Once there you can copy it to the desired location on your network drive (then delete from your desktop). This seems to be more reliable than creating it in place. Once you've done that you can start Time Machine and point it at the drive where the sparsebundle resides and it will find it and start using it. If this still fails, check that the name is exactly right and that you followed all the steps above carefully. Now sit back and relax, knowing that all your hard work is being backed up.

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Reader Comments (192)

ifconfig en0|awk '/ether/{gsub(":","",$2);print $2}'

Would be a much simpler/cleaner method of getting the mac address, avoids piping the command through grep+awk+sed, awk can do it all.

July 23, 2012 | Unregistered CommenterAdam Jacob Muller

comments are also helpful.
Thank you very much!!!

August 13, 2012 | Unregistered CommenterOlmo

Phil, this is an awesome post. I've got everything working as you described on my new setup: Retina macbook pro, airport extreme with a 1TB WD USB harddrive (plugged into the airport extreme), however,

i noticed one thing, and i double checked my old backup drive from when i was using Lion on my imac... everything Time Machine wrote to the backup drive is in a folder called backups.backupdb/<subfolder with computer name>/and then subfolders by date and timestamp... This is how my current MacBook air also backed up to my 1TB backup drive when i backed up over USB...

When i connect the drive over airport extreme though, and use your sparsebundle method, its basically going to copy all 600+GB of data all over again, instead of only updating changed content from the initial backup i did via usb.

suggestions? i'd really rather NOT have to have this thing backup the initial backup of over 600gb over my wifi network.

September 13, 2012 | Unregistered CommenterThomas

also, after following your post, it looks like every time i run time machine now, it tries to create the time machine backup and sparsebundle on my desktop now, even though i moved the sparse bundle to the backup drive and not on my desktip anymore

September 14, 2012 | Unregistered CommenterThomas

Two questions: Does anyone have this working on Mountain Lion? I have a hacked Pogoplug that is sharing out the intended network folder using Samba. Is there something special I need to do in order for Time Machine to see the shared folder? I've got the sparsefile sitting in the root of the shared folder, but TM doesn't seem to "automagically" see it.


December 25, 2012 | Unregistered CommenterEricH


Thanks for saving my bacon!


May 17, 2013 | Unregistered CommenterStacy

It worked :-)

June 11, 2013 | Unregistered CommenterGiron

I got this working. Timemachine sees my WD mybook live now as possible backup drive. The error I get now is "The network backup disk does not support the required AFP features". Does anyone know a workaround?
Using a macbookpro with osx 10.7.5 and up to date firmware on the mybook live. Thanks!

June 19, 2013 | Unregistered CommenterYvette

Thank you very much, this was exactly the info I was looking for. Recently picked up an external drive, got it all formatted, hooked it up to my router aaand nothing. Time Machine didn't acknowledge it at all. Found your post and bodda bing bodda bang bodda boom, Time Machine is doing exactly what I'd hoped. Thanks a bunch

March 26, 2014 | Unregistered CommenterGrateful Post Reader

I went through the post and followed the steps carefully. I was able to create the sparsebundle on my desktop and move it to my network server. When I run Time Machine, however, it will still not find the sparsebundle. I am not very Mac savvy, so it is likely that I have done something wrong or misunderstood what this workaround was intended to accomplish. I have a home network that I set up with MS based computers. I am now using a MacBook Air, but the drive where I want to back up my data is on the home network on a Windows 7 computer. Am I tilting at windmills here? Is there a way to make Time Machine recognize the sparsebundle on G drive on my network server?

March 27, 2014 | Unregistered Commenterpunomatic

I have used this successfully in the past (on the same server) but this time, although I successfully created the sparsebundle, I get this message when I select it as the backup disk: Time Machine Error: The network backup disk does not support the required AFP features.
Thanks in advance for any insights into this problem.

July 15, 2014 | Unregistered CommenterLouise Nadon

Thank you for the clear and effective directions. Really appreciate the help. Rebecca

April 17, 2015 | Unregistered CommenterRebecca Goldberg

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