I updated the MT300A to 2.7 again and the SD-card was dtill not found. A USB stick was however.
I dismantled the unit and with power applied removed and re-inserted the SD-Card.
Now both the SD-Card and USB stick are detected and readable. This is still the case after powering the device off and on again.
So 2.7 does work as long as you remove and re-insert the SD-Card with power on after updating the firmware !
Bit weird but at least it is working now !
Pseudonoise