1st generation iMacs can technically run OSX 10.4, but getting it installed is not easy.
There are several challenges preventing install on a 1st gen “fruit” iMac.
First, there is no DVD drive. I have a retail 10.4 DVD, but the CD installer discs are hens teeth (apparently Apple ran an exchange for a short time, where you could exchange the DVD for CDs).
Second, I can’t boot this model from USB. Even in developer console the USB dvd drive I have (which works fine on os9) is not bootable.
Third, the preferred method for older imacs with no DVD is to use another mac connected via firewire in target disk mode, but the fruit imac has no firewire.
This leaves 2 painful options; setting up a netboot server (not going there) or imaging the install DVD to the 1st partition of the hard disk drive. I decided of course on the latter.
Ultimately I was able to get this going with the help of a couple other guides. The overall process is as such;
You will need:
A newer 10.4+ mac with DVD drive
A USB to IDE adapter, or other way to attach the drive to the above computer
OS 9.2 retail cd
and your osx 10.4 dvd
1. Install a 120GB IDE drive into the CRT iMac (128G hard limit on this model, you can use a 200 but only 128 is usable)
Boot from an os 9.2 installer disc (you can find it; google an iso). Fire up Drive Setup. You need to partition the drive as such:
1st partition, 2900 MB (2.9G)
2nd partition 4900 MB (4.9G)
3rd partition: “remainder” (~100g)
This is because of (at least) the 4th big pain in the ass for these iMacs; osx wants to be on the 1st 8gb of the drive. The partitioning above is quite important for this reason.
2. Close drive setup, and Install OS9 to the “remainder” (3rd) partition. Once this is successful, you can reboot/test etc.
3. Power down the CRT iMac and remove your HDD to prepare OSX.
4. Head over to a newer mac with at least osx 10.4 and a DVD drive. Follow this guide to rip your 10.4 DVD to the new mac. See http://ppcarchive.dyniform.net/cd-less-os-install.html
5. Hook up the CRT iMac’s hard drive to your new mac using a USB to IDE adapter.
6. use disk utility to Restore the 1st partition, using the file you ripped as the image of your OSX 10.4 DVD. I learned about this method from the link in step 4 as well.
7. Once the DVD is imaged to partition #1 of the drive, mount the partition on your new mac. You need to modify one file in a 10.4 installer package so that your older machine isn’t flagged as unsupported. That info is here, so keep this handy:
8. At this point you should be ready to reinstall the drive in your iMac. It should search for and eventually use the 1st partition (mac os install DVD) to install OSX 10.4 to partition #2. If not, use an installer cd to set the startup disk to the install DVD which is now on the hard drive.
9. During the install, skip some printer drivers and languages to save space as needed.
10. Enjoy your dual boot os9 and Tiger Mac!