Pi boot confusion

[gmc]

I've had a pi running for months now with raspbmc - all working fine. This morning is dead in the water - I reboot and only the red pwr light comes one.

I've now taken the sd card and put it in a different pi (Pi-2) and it boots fine so I thought the pi is fried.

I then take the sd card from Pi-2 (running raspbian) and put in in the pi that was running raspbmc and it also boots fine.

So both pi's boot with pi-2 image, the pi-2 boots with the raspbmc image, but the original pi doesnt boot with the raspbmc image!

Huh! I'm confused.
Any ideas what could be causing this? What wont the original pi boot?

I've tried another power supply, and another SD card with the image restored no go. It just refuses to boot with that image.

[MaxK1]

Try squeezing the SD card against the "fingers" of the SD card reader. The pins on the card may not be making good contact. Also, check to see if the SD card itself is slightly warped.

[gmc]

Tried pressing on the card - didn't make any difference.

Now is where is gets even stranger. I left it overnight running on the Pi-2. This morning I shutdown and put the card back in the original and it boots fine.

Queue X-Files soundtrack!