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I have an unused VGA monitor on my desk, so decided to get Mini Display Port to VGA adaptor and hook it up to my late 2009 iMac. On startup, the monitor cannot see the iMac and Display Preferences cannot see the VGA monitor. If I unplug the adaptor from the iMac and plug it back in while the system is up, the connection is made and everything works as expected. The VGA monitor functions properly with a MINI and a G5.

I have to assume this is not normal. Can anyone suggest a next troubleshooting step?

Hmmm... That should work as long as it's a MDP adapter and not a MiniDVI. I have attached a VGA monitor to my iMac before.

Here's the Apple page on these.

https://support.apple.com/en-us/HT201853

I have virtually the same setup (I think mine's a late 2010 iMac) - I bought a proper Apple connector which is a mini display port to vga adapter - a very short lead which connects to the monitor lead. It works fine on my system - seem to remember a few blips where it didn't seem to pick up properly at first - but seemed okay after a few reboots and possibly repair permissions. Pretty sure you have the right connector.

Yes, it is a MDP adaptor, ordered from the Apple Store. Would the DVI adaptor even fit the port on the iMac? And the issue is not that it doesn't work -- it's that I have to unplug and plug it back into the iMac, and then it works perfectly.

Didn't think of permissions. I'll try that.

bayswater wrote:Yes, it is a MDP adaptor, ordered from the Apple Store. Would the DVI adaptor even fit the port on the iMac? And the issue is not that it doesn't work -- it's that I have to unplug and plug it back into the iMac, and then it works perfectly.

Didn't think of permissions. I'll try that.

Let us know - I did have some initial problems like you, but can't remember the exact solution - other than it now works without me thinking about it.

Permissions didn't clear this up. After a while looking for a fix, the only thing that works consistently (other than physically unplugging and replugging the adaptor into the iMac), is to put the system to sleep, then wake it again.

bayswater wrote:Permissions didn't clear this up. After a while looking for a fix, the only thing that works consistently (other than physically unplugging and replugging the adaptor into the iMac), is to put the system to sleep, then wake it again.

Hmmmm... I had some issues with the USB monitor that I use (a DVI is my second) where I had to some combination of restart or re-select or both depending how I started up. Waking from sleep was never an issue.

With 10.10.5, those mostly went away but sometimes... With 10.11, those issues are gone but one day, Chrome and GoogleDocs would not display properly. Then a couple of weeks ago, Google fixed whatever the issue was.

My VGA monitor works when I test it but I sometimes think that the only useful tool for figuring these out is a forked stick used as a divining rod.

mikehalloran wrote:

bayswater wrote:I sometimes think that the only useful tool for figuring these out is a forked stick used as a divining rod.

A variation on the engineer's creed. Hit it with a stick. If it breaks, it needed replacing anyway.