I have a number of devices that are listed in Common Services/DCR/"Devices not configured in ACS."
I can see these devices are not configured in a Network Device Group in my ACS server, which I believe explains the above. This occurred from a bulk import of devices.
If I search Common Services/Device and Credentials/Device Management for a device that is listed in "Devices not configured in ACS" the device is not found.
What is the best way to delete the devices listed in "Devices not configured in ACS?"
The best way is to add those devices to ACS so you can manage them. However, if you do not wish to manage them, the best approach will be to temporarily disable ACS integration. Then you will see the devices listed under Common Services > Device and Credentials > Device Management. Delete them, then re-enable ACS integration, and you'll be set.