So you got a Mac OS X installer disk image (.iso, .toast, .cdr, .dmg) and would like to boot your PowerPC (G3, G4, G5) from it in order to install OSX, but unfortunately, the DVD drive on your old Mac is broken, missing or you're simply out of DVD discs? You instead went ahead and proceeded to boot your PowerPC Mac from an USB stick but it didn't work because the old Mac's USB ports are broken or the Open Firmware command fails for some cryptic reason? Don't have FireWire either? No problem.

Here's one last resort way you can boot your old Mac without a DVD drive or even an USB drive :P

You will need a common SATA hard drive that you can temporarily borrow for an hour.  A hard drive of any size will do, as you typically only need 9GB of space for the whole process.  Note that it will be fully erased, so make sure nothing important is on it before you start.

No SATA port on that G3 or G4?

If you have a G3 or a G4, then it may not have a SATA port.  All they have are IDE ports.  In this case, you should totally get one of those IDE to SATA adapter board.  It's cute, it's small, it fits right in your pocket and it's only 3$.  It's an absolute necessity for any old computer collector or enthousiast, trust me it's well worth it and it will allow your G3 or G4 to use SATA drives such as SSD and even NVMe or M.2 if you chain yet another M.2 to SATA adapter board.

 

If your modern computer runs Mac OS X:

1) Connect the borrowed hard drive to your modern Mac and power it on.

2) Launch Disk Utility.

3) Partition the borrowed hard drive into 2 partitions: make the first partition as 9GB HFS+ and the second partition as the rest of the drive.

4) Use Disk Utility to restore the .iso file onto the borrowed hard drive's first partition (should be the 9GB one). Shut down the modern Mac.

5) Make sure the old Mac is shut down.  Disconnect the borrowed hard drive from the modern Mac and plug it in the old Mac.

6) Press the power on button on the old Mac and immediately begin holding down the OPT (ALT) key until it shows the boot selection screen.

7) Click the borrowed hard drive entry in the screen (should be named something like "Mac OS X Install" or whatever).

8) Proceed to install Mac OS X onto the old Mac's main hard drive as usual.  Then shut down the old Mac.

9) Disconnect the borrowed hard drive from the old Mac and power it back on.  It should now boot from its main hard drive.

 

If your modern computer runs Windows:

1) Connect the borrowed hard drive to your modern PC and power it on.

2) Launch HDD Raw Copy Tool and select the .iso file as the source and hit Continue.

3) Select the borrowed hard drive as the target. Turn off your antivirus as it may prevent to write a boot sector then hit Continue.

4) Turn your antivirus on again and shut down the modern PC.

5) Make sure the old Mac is shut down.  Disconnect the borrowed hard drive from the modern PC and plug it in the old Mac.

6) Press the power on button on the old Mac and immediately begin holding down the OPT (ALT) key until it shows the boot selection screen.

7) Click the borrowed hard drive entry in the screen (should be named something like "Mac OS X Install" or whatever).

8) Proceed to install Mac OS X onto the old Mac's main hard drive as usual.  Then shut down the old Mac.

9) Disconnect the borrowed hard drive from the old Mac and power it back on.  It should now boot from its main hard drive.