Q: Apple supports adding a second monitor to my G4 iMac, but when I check the video card details, it says "Mirror  Off." Is there some way to turn mirroring on? Since Apple claims that mirroring is possible with this model, I assume a change is possible

There is a possibility the display adapter may not be the correct model.

Or that it could be inoperative. Some versions look similar but are not...

Also, the external display has to be one supported by the iMac G4 output.

Mini VGA, to standard VGA, is the primary path.

see search:

https://www.google.com/?gws_rd=ssl#q=apple+vga+display+adapter+imac+g4

Did you restart the computer with the external display attached & turned on?

Sometimes after such an event the extra display will appear in Displays

preference panel.

Depending on the system version running in the computer, the Preferences

panel will also vary; so the answer would to some extent, as well.

Though I have three iMac G4 (17-inch 1.25GHz) computers, none are up

and running; of those one still can, but needs an optical drive. And maybe

by now, also a 3.6v 1/2AA lithium PRAM battery replaced, too.

In the older supported OS X and OS 9 versions (early iMacG4 dual-boots)

the systems are so different as to make precision troubleshooting methods

rather hit-and-miss; usually miss.

The external display was proven to work with a different computer, and it is

a VGA, not a DVI? There was a third-party software that could be used to

help control an external display, and some users found them helpful to set

up a display with older iMac G4,  iBook G4 or PowerBook, with external.

•Switch ResX

http://www.madrau.com/index.html

older version download (shareware) here:

http://www.madrau.com/srx_download/download.html

Then, there was this screen-span utility that was not officially supported

but worked with older computers, to help them with external displays:

http://www.rutemoeller.com/mp/ibook/ibook_e.html & it may be a gamble.

These suggest to add extra ability to change or allow effects to an additional

display; usually if you have working adapters correct to the display & the port

your computer is equipped with, a restart will allow control preferences to work.

Some of the later VGA adapters for MacBook look like the earlier VGA ones

for iBook/PowerBook/iMac G4; so be aware they may look similar but aren't.

Mini VGA to VGA adapter versions can vary, too. I have an old unmarked one

that works with iBook G4 and iMac G4 series; bought second-hand.

Not sure if this will help you much...

Good luck & happy computing!

The process with attaching the display, is to first attach the adapter to the display

then, attach the other end of the adapter to the computer; with it powered off.

with mac off, attach: display + adapter + Mac + restart computer.

This is the most that should be necessary to enable Mirror mode.

Some models use a keyboard command to toggle the display, but some are not

swappable when the unit is running. Those with more active swap options can

be changed over while they sleep, to mirror. Others with extended desktop mode

in their system software, should be accessible by Displays preference panel.

A suggested workaround is Command + F1, although that may work only with

newer Macs or intel-based ones... as stated in this limited bit of information:

http://osxdaily.com/2013/06/27/mirror-display-mac-keyboard-shortcut/

Without a second way to test the hardware, or additional similar adapters to tell,

the issue remains an unknown. Some info may be present in System Profiler on

whatever evidence it finds with items attached; but no way to enable it there.

Another idea may be to use the PRAM reset to see if that helps an different

hardware problem as it may relate to software at almost a firmware level.

•OS X Mavericks: Reset your computer’s PRAM  (method same on older OS X)

Unlikely, but a PMU reset (button under metal bottom plate) has its moments:

•Resetting the iMac (Flat Panel) Power Management Unit (PMU) - Apple Support

Essentially, the workaround is to attempt to determine what is the cause of this

issue where the display does not appear to Mirror. It may be an adapter needs

to fully work, with a correct and functional display, before it appears automatically.

The steps around the campfire are to tell if the heat and light come from using

a match, if the fuel is correct and temperature of ignition source; with air present.

Troubleshooting has led some of my outdoor friends to set up certain electronic

items for 'target practice' at a point when all other methods were deemed failure.

Good luck & happy computing!