Hello,
I successfully obtained an OAuth token for DEVICES_READ and now I try to get list of all devices, but I get empty an response.
Application Id: sq0idp-BfsGZJ_JQbz8-yHj9D3XKA
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Currently the Devices API will only return device information for paired or recently created un-paired device IDs created with the Devices API. It won’t return any device information for any other Square hardware at this time. 
Hello!
I’m happy to see the issue is now solved. Thank you for updating us with the outcome.
Hello @Bryan-Square,
I need to create TerminalCheckouts to receive payments. I found a DeviceId of the needed terminal device in the Dashboard/Settings/Device Management menu. I tried to use Devices API to verify that I can work with this terminal.
Could you please confirm that I am able to use the Terminal API with my real terminal, if I successfully obtained the required access token using OAuth API, even though I can not see my real terminal using Devices API?
If your using Terminal API you’ll first need to create the device code with the API. Device codes created in the Square Dashboard won’t work with Terminal API at this time.
@Bryan-Square, How can I create a new Device code for already paired terminal? Should I press “Forget” for this terminal, create a new code and sign in by using this code? Will this have any drawbacks, e.g. any data loss or some kind of assignment of this terminal to my application? What if I create a Device code using API using my application’s credentials, and then I delete this application, will this terminal continue to work and be accessible by other applications using API?
If you create the device code with the API and then sign in with that device code you’ll be good to go. The Terminal will be available regardless of the application that you use to call Terminal API.
Will this terminal keep all previous settings, and will it be available for payments from Square POS?
With Terminal API you won’t be able to access Square POS. Once you log in with a device code it will switch the Terminal to the Terminal API mode for taking payments and any settings in the device will not be relevant.
So, you are saying that a terminal device can be either used from Square POS, or from Terminal API, and not from both. Is that correct?
Yes, that is correct.