I've been a Mac man since '98, and have always used an external monitor to expand the size of my desktop, but I've never had this problem before, and don't know what to do. I will try to give as much info as possible, and appreciate any suggestions.

The equipment involved includes a late 2006 17" MacBook Pro, 2.33 GHz Intel Core 2 Duo running Snow Leopard 10.6.8, and an Acer x223w 22" external LCD monitor connected with a VGA cable and adapter. The MBP native resolution is 1680x1050 and the Acer native resolution is 1600x1000 (both are the same aspect ratio of 8x5). Also involved is an external HD containing the HD that was in my older G5 tower that suffered a sudden death from a leaking cooling system in 2010, with OS X 10.5.8 on it. For future reference, I will refer to the MBP running 10.6.8 as "MBP HD", and the external G5 HD running 10.5.8 as "G5 HD."

Everything was working fine for almost two years, until about a month ago.

For at least 95% of the time I use my Mac, I'm running off of the MBP HD, but I occasionally switch over and restart on the G5 HD to run some programs, and then restart back on the MBP HD when finished, and have never had any issues. This past January I did that one day and as it was restarting on the G5 HD, I noticed the external monitor had no color, and was very darkly contrasted. I didn't really care at the time because I was only going to be on the G5 HD for a couple of minutes. When I finished what I was doing and restarted on the MBP HD, the external monitor was fine. The next time I had to restart using the G5 HD a week or so later, the external monitor had the color working fine, but was in the wrong resolution. After it finished restarting, I went into System Preferences to correct it, but the Acer native resolution wasn't available for the external monitor, nor was any other resolution using the same aspect ratio (8x5), so I gave up and finished my business. When I then restarted using the MBP HD, the issue followed me and the external monitor was in the wrong resolution still. I went into System Preferences, and had the same issue there as well; the native resolution for the Acer wasn't even an option, nor were there any other choices with the same aspect ratio, and it's been that way ever since.

I ended up selecting 1600x1200 and dealt with everything being squished, but I'm not content with that anymore. I need to figure out a way of getting my Mac to recognize the native resolution of the external monitor.

I searched hi and low for drivers for the Acer, but none exist for Mac's. This never mattered before because I guess my Mac determined what it should be on it's own, but it's lost that ability. I would be perfectly happy with the MBP's native 1680x1050 resolution, but it's not an option for the external monitor, just the MBP's built in monitor.

Also worth noting is that I upgraded to Snow Leopard on the MBP HD about 4 or 5 months ago, but again, this issue didn't start until about a month ago.

So I guess my question is; how can I add other resolutions to the System Preferences Display panel for the external HD, or better yet, get my Mac to recognize them and add them itself like it always did before? Other things I've tried have been disconnecting and reinstalling the monitor, repairing Disk Permissions, running MacKeeper, and even tried going back into the System Preferences on the G5 HD to see if I could fix it there, but no luck.

Also, the CD that came with the Acer is long gone, but I had never needed it it in the first place, so don't know if it would have helped.

Thanks in advance for any answers, advice, or suggestions. If I overlooked something, please feel free to ask, and I'll do my best to promptly answer any questions asked.

Take care,