Flashing and debugging of externally compiled program
>> The rest of type string is not interesting in my opinion because describes mcu package. It is not related to debug configuration:)
The complete part number should be specified
I have tried using STM32F429I-DISC1 (ST-Link V2-1) and STM32F3Discovery (ST-Link V2) and it works
Just a comment:
to use STLink V2-1 you should include this in your custom.cfg
source [find interface/stlink-v2-1.cfg]
Note: When creating a board, you can specify ST-Link version, After that SystemWorkbech will create the custom.cfg for you 
This kind of message can appear if
- You have multiple running debug instance and both of them are listening to the same port number (3333)
- You device flash memory is protected
Genarally it is the first case, so try to close the running debug session and try again (from Debug view : Terminate/Disconnect All).
Also restarting SystemWorkbench will do the Job too.
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