Ensure you are flashing a firmware file specifically for the DSO2512G and not its smaller sibling, the DSO1511. Conclusion
Check your cable or try a USB 2.0 port instead of USB 3.0, as some older bootloaders have timing issues with high-speed ports. dso2512g firmware
(Windows only). Use Zadig to replace the generic driver with WinUSB or libusb . Ensure you are flashing a firmware file specifically