I asume that the 256Gb drive is a SSD drive?
Did you install it yourself or is this one out of the box?
If you suspect the new memory for causing the problems you are having, I would disconnect the external drive and revert back to the original memory.
Boot up and check to see if the error message is gone.
If so, shut down the mini and install the new memory.
Boot up and check again...
Only reconnect the external drive when the issues have gone, if they return after rebooting, the drive coulf the problem in this case.