Storing a secure key in an embedded device's memory
An issue with storing a key in RAM is, there are attacks that involve erasing the flash to unlock the device and then using a debugger to copy the RAM contents. Problem with internal EEPROM is the same as RAM unless you can lock it. External EEPROM is worse, unless you encrypt that.

My druthers would be store the key in flash and then lock the lock the processor to disable debugging access. If possible use a separate key for each and every device. If you copy the key to ram, make sure it's gets over-written when done.