Your question is confusing. As far as I know the nRF51822-mkit only shipped with mbed interface firmware. I'm sure how you got this board to come up as JLINK. Can you please provide more detailed steps about what you have done. Thx!

Thanks @Sam, yeah I was confused too - I just plugged the brand new board into my mac and the USB volume that mounts is named "JLINK". I copied a compiled simple blink code example and copied the hex file to that volume then reset and nothing happened. I noticed in discussion about the (probably unrelated) Yosemite USB issue that people were copying images onto the BOOTLOADER volume (which mounts if you hold down the reset button while connecting to USB). That didn't sound right to me but (foolishly) I gave it a go anyway. I assume what's happened is that the bootloader has been wiped and replaced with my led blink example which won't work very well as a boot loader…

So I presume I need to re-flash the boot loader using a segger board or jtag or somthing, but this is my first mBed board so I don't really know anything! Any help appreciated.

I'd imagine that the boatloader cant over write itself, rather only overwrite the debug app in this case JLINK. Does your board have a mbed-enabled logo on it? Just trying to track down why the JLINK app came default on the board and which bootloader is installed. Thanks for the details.

@sam yes it has an mbed logo - it's the new nrf mbed dev kit from Nordic.: http://developer.mbed.org/platforms/Nordic-nRF51822/

Actually sorry it's not quite that board in the link because it has four buttons. It has a sticker that says V1.1.0 so I guess it's a slight revision upgrade of that one.

I'm in the same boat with one of my boards which I bricked trying various workarounds for the Yosemite USB issue. Mine is also an nRF51-DK and it did ship with JLINK software on it and I flipped it to the MBED using the mount workaround.

However I've had lots of problems with stability with that workaround and eventually copying over a binary (I thought to the flash, not the bootloader) I've bricked it in much the same way as Mark has done. I get no LEDs whether pressing the reset button on restart or not, I don't see the mass storage device at all under OSX nor under windows, neither the normal one nor the bootloader volume. I'm assuming at this point I've entirely destroyed the bootloader and need to load it back onto the controller chip from scratch.

So the question is, if you have no bootloader, or no working bootloader, such that the device doesn't mount as a mass storage device at all in any way, how can you reload a fresh bootloader back on to it. Is there a way to do it via the serial interface or any other low-level programming method or do I toss this board in the bin and buy a new one?

FYI for @Roland, I created a question at the Nordic devzone forums: https://devzone.nordicsemi.com/question/29109/how-to-reflashun-brick-my-nrf-dk/

@Mark thanks I saw it - which made me start googling again to see if there was an unbricking answer I missed when I looked a couple of weeks ago just after I messed up mine, and then I saw this and posted a 'me too' reply. I assume there must be a way to get a bootloader onto a clean chip, someone does it during manufacture for a start.