Having my iMac setup for TimeMachine I wanted to do the same for the new Macbook Air. While I don’t want to run around with TimeMachine Backup drive all the time (used to do that for my last MacBook, bad decision) I wanted to setup TimeMachine Backup to my iMac on the same external HDD.

Here’s the link that helped me doing that: Ease Time Machine networked backups

The workflow:

On the iMac

1. Connect the TimeMachine Drive
2. Under System Preferences, Sharing share the drive

On the Macbook Air

1. Connect to the shared drive
2. Setup TimeMachine to use the drive
3. Start a backup
4. When “Preparing backup …” turns into the actual backup, Cancel
5. Mount the newly created sparsebundle file by double-clicking
6. Open DiskUtility and select the sparsebundle file
7. Select the Partition tab
8. Shrink the size of your Time Machine destination as needed
9. Click Apply

Alright, now the Macbook Air is backing up to the Disk via the sparseimage file and over the network. Now the tricky part begins. Getting the iMac to backup to a sparseimage file locally won’t work. You have to follow these steps:

On the Macbook Air

1. Connect the external drive to the Macbook Air
2. In System Preferences select Sharing
3. Turn on File Sharing and select the external drive to be shared

On the iMac

1. Connect to the newly created share and navigate to the Time Machine drive
2. Open Time Machine preferences and select the Time Machine Share
3. Let Time Machine create another sparsebundle file for the iMac
4. Once that’s done cancel the backup again and mount the bundle to shrink it (see top)

On the Macbook Air

1. Turn off File Sharing to disconnect the drive
2. Connect the drive to the iMac again

On the iMac

1. Open Time Machine Preferences
2. Choose Backup Disk and select the local disk
3. Happy Time Machine’ing! The sparsebundle on the disk will be used even though it’s attached locally!