I looked and found no answer so far. They everywhere tell you "...you must not...", but nobody says WHY?

I have a server that I would prefer to stay up, no rebooting and booting (like windows) with knoppix and parted to do a complete backup over the net.

So, what will happen if I do a partimage without unmounting a partition?

Is there another backup tool that would backup one partition to another machine over the network, without restarting the machine every time?

Any way I can remount the root partition read only for the time it takes to take the backup?

I would imagine you can't do it on a partition mounted read/write because it would be possible for data to be written/changed while it is making the image, and that wouldn't work so well. You could end up with all sorts of file corruption.

You can, however, rsync your important data onto another computer. Now this doesn't provide quite the same level of back, because you wouldn't just be able to restore the entire partition like you can with partimage, but at least you'd still have your data.

... if I remember correctly (else man mount ).

Will work only in the case, that you have no processes which want to write (logs etc.) to the disk.

Hmm, can I do it (remount ro) or will it not let me?

I would not mind some log processes scream hell at me while I steal away the backup.

Well, anyway, thanks for the pointers. I will look into the rsync.

You aren't going to remount your root partition unless you can basically shut down all services. If this is a mission critical system something else you might want to look at is a Mirrored raid array. Linux provides good software raid tools so you don't have to purchase a raid adapter. It also has drivers for 3wares IDE raid adapters which I like a lot. They allow you to setup a hardware based raid array without breaking the bank.