time machine to HD on network?

Using Time Machine over a networked drive it is not recommended or supported by Apple. The software doesn't stop you from configuring it that way, so many people do it anyway. If you do a search in Apple's Support secion you should find a technical document about it, or there is an excellent article here:

http://pondini.org/TM/Airport.html

It isn't supported by Apple, but there are plenty of websites showing how to get it working. Some people claim that because Apple doesn't go out of the way to prevent you from doing something, then they support it. I don't buy that.

In any event, you would not want to use the free space on an existing drive for Time Machine. Time Machine needs its own, dedicated partition. You would have to repartition that drive to use it with Time Machine. Again, you could make it work on a single partition, but you will regret it later.

Contrary to what's stated above, backing up to an AFP-shared drive attached to a Mac is supported by Apple:

OS X Lion: Disks you can use with Time Machine

And the answer to your question is that TM will automatically create a disk image file to hold the backups of each client. However, you need some way to back up the other data on the drive. You can't back it up to itself.

This Apple article specifically states that you can't use a drive connected via Airport:

http://support.apple.com/kb/HT2038

I read the article posted by Linc Davis. I must admit it is open to some interpretation because of how it is worded, but I read it differently. It says: "Time Machine can’t back up to an external disk connected to an AirPort Extreme, or to an iPod, iDisk, or disk formatted for Windows."

He must be interpreting it to read "Time Machine can’t back up to an external disk connected to an AirPort Extreme disk formatted for Windows.", which might imply that it CAN be connected if it is formatted for Mac. But given all the other (and more specific) articles published by Apple and elsewhere on the subject, I don't think that is the correct interpretation.

This would be a VERY big deal if this was now supported by Apple, so I can't imagine that they just suddenly changed their minds and decided it was now supported without any announcement of new features and without it being part of any major software upgrade. It is more likely that the correct interpretation of that sentence is that those 4 items listed are to be read separately, as in:

"Time Machine can’t back up to an external disk connected to an AirPort Extreme."

"Time Machine can’t back up to an external disk connected to an iPod."

"Time Machine can’t back up to an external disk connected to an iDisk."

"Time Machine can’t back up to an external disk connected to a disk formatted for Windows."

You have, I gather, original data on that drive now. Like all data, it has to be backed up, or you'll lose it when the drive inevitably fails (if not sooner.) Backing up to the same drive gives you no protection from a drive failure.

The OP doesn't say that his drive is connected to an AirPort Extreme. He says it's connected to an iMac. I was answering the question he asked, not a question he didn't ask.

Everything except cache and log files, and whatever else you choose to exclude.

You should partition it. TM does a better job of managing space if it has a volume to itself. Don't back up your backups.

Linc Davis wrote:

The OP doesn't say that his drive is connected to an AirPort Extreme. He says it's connected to an iMac. I was answering the question he asked, not a question he didn't ask.

The title of this post is "Time Machine to HD on Network?" so naturally I assumed he was talking about... well, just that... backing up Time Machine to a HD on a network. Perhaps the OP should clarify.

Antonio Conte wrote:

Hi that's right, the external 2TB HD is connected to an iMac which in turn is connected to a network. Which we can all mount as a drive on our machines. TM appears to be able to see it but I didn't want to go any further as I am not sure if it will affect the HD in any way?

Right... got it. Sorry, the title made it sound like the HD was connected directly to the network as opposed to being on the network via the iMac. I don't have any experience with that configuration so I won't be able to help. Personally though, I feel that when it comes to backups, I don't want to take any chances. If there is any doubt, I would never risk the very thing that is meant to keep you safe. Good luck.