I have just opened the NUC and it has a very short mSata installed. Digitec has larger ones in this format in stock. My question is, I currently have an additional 1TB HDD installed, which I would like to keep installed. The mSata SSD has two connectors A and B in addition to the mSata connector at the front. Can I just change them like that, because I don't see these two small plugs A and B on the other mSata pictures.

If it is a normal SATA SSD then it should work. Here is a good video that explains how to copy the files: https://youtu.be/EVDTNpI8f_...