Q: it says the 27inch Mid 2010 model can be used as a display, is this ALSO correct for the 21.5inch Mid-2010 model? thanks

The Apple article on Target Display Mode says that capability for 21.5-inch iMacs did not become available until the mid- 2011 models.

Is this the support page you found:

Use your iMac as a display with Target Display Mode - Apple Support

Well I have two iMacs in front of me. One a 2009 21.5" iMac (W8********5PC) and the other a mid 2011 27" iMac (C0********HJW).

Newer 27" is the server

Older 21.5" is the host.

Tried Thunderbolt and Mini Display - and NEITHER work.

I was on tech support twice for this.  Both assured me it WOULD work with the Mini Display cable - but it doesn't.

Frustrated

<Edited by Host>

Sorry you are frustrated, however the bottom line is it will not work  If you have not reviewed this, it is a good read: Use your iMac as a display with Target Display Mode - Apple Support this document verifies only late 2009 27" iMacs support TDM, no mention of 21.5" models is made because TDM isn't supported on those models.

It definitely does not work.

I just purchased a used used 27" mid 2011 iMac and already had a late 2009 21.5" iMac hoping to go Dual Monitor.

Tried Mini Display and Thunderbolt. I even tried to use my Macbook (mid 2010) to see if that would work, but no combination would allow the Dual Monitors.

_glen_ wrote:

 

It definitely does not work.

I just purchased a used used 27" mid 2011 iMac and already had a late 2009 21.5" iMac hoping to go Dual Monitor.

Tried Mini Display and Thunderbolt. I even tried to use my Macbook (mid 2010) to see if that would work, but no combination would allow the Dual Monitors.

Please read the Apple article from my post above. Apple is VERY clear it will not work on 21.5" models from 2009 or 2010.

I'm probably going to be sorry I asked this but there seems to be some ambiguity with the chart from the article (to me at least).

Does the chart with the iMacs listed allow for TDM for model year to model year only (i.e.2011 to 2011) or is it backwards compatible as long as the model specified is on the chart?

The simple fact is a 2009 21.5iMac does not support TDM. There is absolutely no ambiguity in the chart, it clearly states 27" 2009 iMacs are the only model from that year that supports TDM, there is no mention of a 21.5" for a reason.

Only the specific models on this list can use TDM. Got it and thank you.

As long as it is on that chart then they all can use TDM among the models on the chart (with one of the cables listed).