IMac – Use old iMac G5 as external Mavericks-style display

There are a couple different approaches you can take.

Approach 1 - Erase the newer iMac, reinstall 10.6.8 and use migration assistant to migrate your Time Machine data

With the newer iMac:

1. Insert your Snow Leopard DVD (not the gray disc, a retail copy) and boot to it either by holding C while powering on or holding option while powering on and then selecting the 10.6 DVD.
2. Select "Disk Utility" from the Utilities menu in the Snow Leopard installer
3. Select the iMac's hard drive from the list on the left in Disk Utility, click on the "Erase" tab, type in a name for the hard drive (Macintosh HD?) and click erase.
4. Once the hard drive is erased you can close Disk Utility and proceed with the installation of 10.6, following the on screen prompts.
5. Once the installation is finished your newer iMac will boot to the normal welcome to OS X screen.
6. I believe the third or fourth step in setting up the newly installed 10.6 will ask you if you want to transfer information from an existing Mac. Make sure your external Time Machine disk is connected and choose the "From a Time Machine Backup" option and follow the onscreen prompts.
7. Once the migration is finished you should be able to login with your normal user account and your data should appear to be just as it was the last time your old Mac backed up.

Approach 2 - Clone your older iMac to the newer iMac

This requires that you have a firewire cable

With your older iMac:

1. Download Carbon Copy Cloner on your older iMac
2. Power on your newer iMac while holding T, you should eventually see the Firewire symbol (sort of looks like a Y) floating around it's screen
3. Connect the two Macs together via a firewire cable, your newer iMac should appear as an external disk on your older iMac's desktop
4. Open Disk Utility (/Applications/Utilities) on your older iMac
5. Select the newer iMac's hard drive (should be an orange icon) from the list on the left in Disk Utility, click on the "Erase" tab, type in a name for the hard drive (Macintosh HD?) and click erase.
6. Once the newer iMac's hard drive is erased you can open Carbon Copy Cloner
7. Follow the on screen prompts in Carbon Copy Cloner to select your newer iMac's hard drive as the destination and then clone your older iMac's hard drive over to it.
8. Once Carbon Copy Cloner is finished you should be able to boot your newer iMac and browse through to see all of your older iMac's data now in place.

Hopefully one of those options works and you're able to get that data migrated with little to no pain.

I ran into the same issue, and finally figured out the trouble is related to the CoreStorage partition type that is shipping with Yosemite.

To solve the NetRestore issue, first boot into a recovery mode (either the built-in recovery drive, an external USB drive, or net recovery).

Open up Terminal, and run these two commands:

diskutil cs list
diskutil cs revert A-Long-string-of-hex-values

The first command will print a tree view of the CoreStorage drive. Of note, you should see a few long UUID strings. Likely there would be one for "Physical Volume", "Logical Volume Family", and "Logical Volume". It is the string after the last one ("Logical Volume" which you should copy and paste as A-Long-string-of-hex-values in the second command.

See http://awesometoast.com/yosemite-core-storage-and-partition-woes/ and The OS X Man Page for diskutil for a bit more info. (Note that some sources may use "coreStorage" in place of "cs" in the above commands. According to the man page, the two are equivalent)