Sharing External Harddrive

Since it should be a goal to backup up continuously, with no human intervention... the most logical way would be to put the drive on your network.

With just an iMac... you can leave it continuously connected. Because of the mobility of your MBP... it makes more sense to have it on the network... and then both machines can access it for backup.

/Jim

 

If your wireless access is on the same network as your iMac, then you can wirelessly backup your MBP too. You don't need to partition the disk unless you want to limit the maximum "growth" of either backup set. Time Machine backups will slowly grow to consume all the disk space they have available, they they will start discarding older versions of files which have been updated. That could "starve" the other backup sets for disk space.

If you want to use your iMac as a Time Machine server with either a internal or external disk, you will need to leave it turned-on all the time that a backup request might be coming from a remote computer. I think you will also need to be running the "OS X server extensions" to share a drive for Time Machine usage, but this is only a $20 upgrade for Mountain Lion.

You can also get a NAS (Network Attached Storage) device to serve as a whole network backup. There are many brands and sizes of NAS to choose from. The Apple "Time Capsule" is commonly used for this and WiFi. If you have a Apple "Airport Extreme" for your WiFi, you can add a USB disk to that for backups too.

The only way to use a directly connected disk drive (USB, FireWire, Thunderbolt) with several computers is to physically eject, unplug, and move the disk to the other computer. Your Time Machine backup sets will be named for the name of each machine, so will not conflict.