Backing up networked iMacs I have an iMac connected by wire to an external (non-Mac) hard drive which is used to store back-up images via Time Machine.  I want to buy a second iMac and network the two together.  As far as I understand it, this will mean that (subject to permissions) any user can sit at one of the machines and see/use the contents of both hard drives.

How do I now back-up both iMacs to the same hard drive? I can't afford to buy the expensive Time Capsule.

Suggestions will be greatly appreciated.
You can use one USB drive to back up multiple OS X Macs. Just connect the drive sequentially to each. 

You could cycle the drive daily, weekly, monthly or hourly.

Each backup is kept separate and yes, anyone can read the backup files from either computer if they are an administrator of either Mac.

You could also make use of Crash Plan software to back up each Mac to the other over the network. That way you could have Time Machine on one Mac cover both of the computers - one directly and the other backing up the crash plan data.