failed to start debuuging on STM32F0308-Discovery board
Hello,
Are you sure all debug launch are terminated, everything clean ?
Then, i don't know, can you try to program the target elf directly ?
Right click on Demo F030 prj and select menu Target => Program chip.
And try Target => Erase chip also if prog not working.
If this is still not working, can you try to do a mass erase with another tool like stlink utility on windows.
Rgds,
Laurent