Just changing your computer would have no effect on the operation of the AirPort Express to the point where it would flash its fault light. I suspect that the problem has nothing to do with your Mini and everything to do with your AirPort Express (and possibly a change in the data being provided by your cable modem).
Try plugging your cable modem into the AirPort Express which you have had there (not the one you were using to "extend" your network) and then connect to the AirPort Express with your Mini. Can you do that? If you can, is your Mini simply failing to connect to the Internet via wifi?
Have you contacted your cable provider to be sure you have your AirPort Express set up correctly? It's possible that you need to set it for "bridge" mode; or not set it for "bridge" mode; or any of a number of other configuration possibilities that your cable provider might have recently changed.
Yes, I didn try using both Airport Expresses connected to the cable modem, with the Mac mini just a couple of feet away. In both cases the lights blink amber, and Airport Utility does not see them.
A little more explanation: The reason I own a 2nd Airport Express is to a different room to powered speakers connected via the Express's audio port, so I could stream music from my Mac & iPad. So the 2nd AE was not involved in feeding data to my Mac at all. This 2nd AE also blinks amber now no matter what -- when I tried connecting it to the cable modem (in case the 1st AE had died on me) and now when it sits by itself connected to my speakers in the other room.
Because I was using a 6-year-old iMac before and Snow Leopard I wondered if the AEs were working with an older firmware and just won't run under Mountain Lion....
The AirPort Expresses don't know or care what OS you're running on your computer. The blinking amber lights indicate a failure to connect to the network or to the Internet, or to some other fault; it has nothing to do with either of your Macs or your Mac's ability to connect to the Airport Express.
If the lights began blnking amber when you connected the AirPort Express to your cable modem, I would suggest the fault is there and again you might consider contacting your cable provider for some configuration assistance. The first step, though, will be getting into the configuration options of the AirPort Express.
Don't worry about firmware; just connect to the first AirPort Express using Ethernet to find out if AirPort Utility will detect the AirPort Express that way. That will allow you to make whatever changes are necessary. I suspect, however, that you will not be able to do so because the Mountain Lion version of AirPort Utility will not work with the oldest versions of the AirPort Express. You will need an older version of AirPort Utility:
1) Download the Lion Version of AirPort Utility from
2) Download a free tool called unpkg from
3) Use unpkg to extract the application from the .pkg installer inside the dmg image you´ve downloaded and drag it into your Applications folder.
Please note that the older version of AirPort Utility is needed only to set up your AirPort Expresses, NOT to connect to them as wifi clients. Just get them set up and then ignore them as you used to do.