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I have never needed to do what you describe (I have always unpaired from the old computer) but you could try a couple of things:
Turn the device off.
1. Delete the device in system preferences>bluetooth.
2. Trash a preference file (com.apple.bluetooth.plist)
3. Reset the PRAM and SMC (instructions in knowledge base articles).
Turn the device on and restart and see if it works. If all else fails, an Apple store might be able to help if you are near one.
Well, I have a solution that worked for me. This is on a MacPro and not a MacBook but presumably it would work there too. Maybe not as it has a keyboard built in but maybe.
The deal is you have to unplug ALL usb devices, have your wireless keyboard and mouse turned on and restart. The system see's there are no input devices and searches for them. Then asks you to type a passcode and hit return.
The first time through I waited for the box to come up to type the passcode in to and while I was waiting it failed. I tried again and typed the passcode and hit return without waiting for an entry box and it worked! I took my friends aluminum keyboard back to him and tried the same procedure on his iMac and it worked there too!
Our keyboards are working again.
Taken from :- Topic : Apple Bluetooth Keyboard won't pair
That was a fantastic suggestion! I have been reading several posts tonight because my Mini wasn't recognizing my Trackpad. Doing your suggestion of starting up while the trackpad was in discover mode did the trick. The Mini said it was having troubles connecting, and asked me to click continue if I was serious (I am paraphrasing, of course!). No all is well!
Unfortunately, I am having the same issue, and I have NOT been able to find any information online that details how it might be possible to force the Apple Wireless Keyboard (2009) into pairing mode, if it is convinced that it has been paired with anything.
I had previously paired the keyboard to an iPad, and now that I have removed the pairing from the iPad, I would like to pair it again to do some testing, but now the keyboard, which had previously appeared as "Apple Wireless Keyboard" only appears as "Keyboard" for about 3 seconds before the green light on the keyboard turns off (right about when I expect to see the light flashing to indicate it is ready for pairing).
The iPad is the only device to which I believe this keyboard has ever been paired. With other Bluetooth keyboards I've received along with a computer, they've been pre-paired ahead of time, so I know I need to remove the pairing before they are ready for use with any device. In this case, I'm afraid that under-powered batteries on the last pairing have gotten the keyboard stuck thinking it is paired, when no other device is actually listing it as a paired accessory.
I would love to find a post that details being able to use a key combination when starting the keyboard to force it into pairing mode, "forgetting" all previous connections, but I haven't found anything yet.
I had exactly the same problem - I figured out how to force it into pairing mode, though its not intuitive and kind of a pain. With the keyboard off, you hold down the power button on the keyboard and the LED will start to blink, do not let go of the power button. Open up the bluetooth add device wizard, and the keyboard will appear there as long as you're holding down the button. Select the keyboard and pair it, when it says "Connecting" you can let go of the power button.
Ohh thank you SO much my friend, this did it for me too.
I think my trouble started when I paired it with my MacBook Pro (the mac I'll use it with, due to cola-spill) and then tried pairing it with my iPad, just to see if it could work (it can, works great).
After that I wanted to use it back on my MBP again, , but it had forgotten the pairing, so i tried No luck until your awesome and rather unorthodox suggestion (would never have thought of that myself
didn't work for me. Holding down the power button on my aluminum BT keyboard won't force it into pairing mode. I've un-paired it from every other mac in the house and all the ipads, but it still won't go into pairing mode (blinking green light).
I've tried removing the batteries and letting it sit for an hour, to see if that would wipe the keyboard's memory, but no luck.
Anybody know how to force the KB into pairing mode?
I tracked down this thread after a couple of days of going back and trying to solve the issue. I was with Mark, where doing everything still had failed, but then I succeeded.
The most hopeful tip was the one above, about holding down the power button until you see the Connecting... message, but as written it didn't quite work out. Here's the steps that did work:
With Setup Bluetooth Device not open, press and hold the power button on the aluminum wireless keyboard, paying attention to the fact that the green light does come on (otherwise you're basically holding it in the off position!).
Select Setup Bluetooth Device from the Bluetooth menu icon. The keyboard is certain to be listed.
While still holding that button down, click Continue.
Don't let go when you see the Connecting message!
Soon after, the set of numbers for you to type should appear. Now you can let go of the power, and the sync up should work correctly.