This is really strange! When I have my EFM32 Giant Gecko Starter Kit connected, my PC will not boot! When I power it on, all I see is a black screen with a blinking cursor. Everything is fine when I unplug the STK, so I have a workaround, but I would like to know what is happening?
Thankfully, there is a simple explanation for this.
As the article EFM32 STK shows up as a Mass Storage Device explains, ARM's online IDE for their mbed OS does not have a way to program the flash on a target device. They solve this problem by having the board controller that manages the USB debug interface present a USB mass storage device to the connected PC. Files dragged/copied onto this storage volume are programmed into the flash of the target device.
Needless to say, this USB mass storage device does not hold anything that looks like a bootable file system. Consequently, if your PC...
is connected to an EFM32 Starter Kit that supports mbed (which is nearly all of them), and
has the "Boot from USB mass storage device" or similarly named BIOS option enabled,
...then, at power-up, it will attempt to boot from the file system that the EFM32 board controller provides to support the mbed OS.
Depending on how gracefully your PC's BIOS handles USB storage media without a valid file system, you may see anything from an error message like "Boot Device Not Found" to the black screen with nothing more than a blinking cursor described above. Again, simply powering up with the EFM32 STK disconnected allows your computer to boot normally.