Bluetooth dongle conflicts with internal BT ??

Some thoughts: both BT radios would be transmitting on the same 2.4GHz frequency, so could be interfering with each other. And you have the two BT radios relatively close to each other. Perhaps use a USB extension cable to move the external BT dongle farther away from your mouse & keyboard. And closer to your solar panel data device. I think that most BT radio "crank up the power" in weak signal conditions so if the solar device is far away, then the BT dongle may occasionally be "louder" than the iMac's BT radio. Thus the idea of moving the BT dongle away from the iMac and closer to the solar.

I do something simiar to you with a Mac Mini and a BT dongle on the USB port of my monitor. I used to have the dongle plugged directly into the Mini, but after a bunch of troubleshooting I moved the dongle to the monitor. This appears to put the BT dongle far enough away from the Mini to not cause interference.

...but when I close down or start the iMac again mouse and keyboard drop dead again...

The other possibility I'm thinking of is that the mouse and keyboard are trying to connect to the BT dongle instead of the internal BT radio. This could be more likely if the dongle is Class 1, which has a stronger radio. BT has a range of 10 meters for Class 2, which is what is in the iMac, and 100 meters for Class 1. You won't find a 100 meter USB extension cable, but hopefully moving the dongle just a few meters will work.

Otherwise, USB mouse and keyboard will obviously not have Bluetooth issues. 😉

Hello Carnut, Did you solve the problem? I have just installed an SMA solar panel system and have experienced exactly the same problem that you describe. I've moved the BT dongle as far away from the Mac as I can (only about 2 metres), and even tried shielding it with a metal can, but the Mac's BT still packs up as soon as I plug in the dongle. I would be very interested to know if you found an answer. Thanks.

Mac's that have built-in Bluetooth don't support external Bluetooth Adapters.

The OS does not have any way of allowing you to switch between internal and external Bluetooth, so doesn't know whcih one to use.

The only way to successfully use an external Bluetooth dongle would be to somehow completely remove/disable the internal Bluetooth.

It would be like connecting to mice to the same computer and 2 different people using them at the same time. The computer would not know which mouse should take precendence.

Mac's that have built-in Bluetooth don't support external Bluetooth Adapters.

False (for current Mac Pros at least)! In late 2010 Mac Pros (and presumably beyond) plugging in a BT dongle disables the internal BT. I use a BT dongle and verified the internal is disabled with Bluetooth Explorer.

For Mac Pros prior to 2010 based on discussions on this topic elsewhere it appears the dongle doesn't disable the internal so the BT board needs to be removed. I have no idea what happens with the portables.

the same thing is happening on my 2011 MBP internal is disabled, and that is not good, as I need 2 bluetooth adapters, one for Mac os x and one for Parallels Desktop... big time bummer...