How to use my iMac as a monitor for my Mac Mini?

Its not going to work ever through Firewire. Firewire connections transfer data only, not video.

While its possible, its not easy, and you need additional hardware to convert the DVI video signal from the Mac Mini into a Thunderbolt / MDP signal the iMac can use for Target Display mode. The adapters that did this are old and no longer being produced.

Its going to be much simpler to find a DVI display than to attempt to jury rig anything with the iMac at this point.

click here ➜ Use your iMac as a display with target display mode - Apple Support

and here ➜ Dual Link DVI to Mini Display Port Converter

and here ➜ https://forums.macrumors.com/threads/target-display-mode-on-ppc-macs.2240855/

The 2009 iMac must be a 27-inch model to work with Target Display Mode (TDM). There is no way to make TDM work on a 21.5" iMac for 2009-10. Only from 2011 through Mid 2014 were 21.5-inch iMacs supported.

A G4 Mac Mini can run no higher macOS version than 10.5 "Leopard."

Ollie_Obo wrote:

I have an early 2009 iMac, running El Captan. I want to connect an older MacMini G4 to the iMac to use as a display. .....

I don't recall if the G4 versions of OSX supported screen sharing but you could possibly use screen sharing over a network and with Macs that old, it may be just fine.

Other than that, just get a cheap TV or display and get what ever adapters are needed.

Ollie_Obo wrote:

Appreciate the reply. As a go around, how would I connect the mini to the Mac, using the mini as an external disc, and can be seen by the iMac. I could then transfer my music to the mini. I don’t want the mini connected permanently, just to transfer the data.

As mentioned by Jan above, Target Disk Mode should work over Firewire, and let you add any files to the Mac Mini from the iMac.

click here ➜ Transfer files between two Mac computers using target disk mode - Apple Support

It has the benefit of not actually need a display connected to the Mac Mini to set up only a keyboard so you can hold down the T key when you turn it on.

>how would I connect the mini to the Mac, using the mini as an external disc, and can be seen by the iMac.

Hi,

The basic FireWire target disk mode method is described here:

Transfer files between two Mac computers using target disk mode – Apple Support (UK)

https://everymac.com/systems/by_year/macs-released-in-2009.html

https://everymac.com/systems/apple/mac_mini/index-macmini.html

If that Early 2009 iMac is a 24-inch model, it can run in Target Display Mode, using Mini DisplayPort input. (Not Thunderbolt input. Thunderbolt wasn't yet around.). Whether it would accept Mini DIsplayPort output from the older Mac mini G4 is anyone's guess.

If that Early 2009 iMac is a 20-inch model, it doesn't support Target Display Mode. 21.5" iMacs appeared in Late 2009, and they didn't support Target Display Mode, either.

Use your iMac as a display with target display mode - Apple Support

Both the Early 2009 iMacs and the PowerPC G4 Mac minis had USB 2.0 ports. The PowerPC G4 Mac minis (2005 and Late 2005) came with 40 or 80 GB, 4200 rpm, hard drives.

So another possibility would be to transfer data using an USB drive. Although I would not recommend flash drives for long-term storage, 128 GB USB flash drives that could hold everything from the old Mini's disk don't cost very much these days.

Addendum

The T key is the key, so a (wired USB) keyboard will be needed for the Mac mini.

Additional information about FireWire may be found under Mac Help on the iMac.