It was not that difficult, although I made a mistake...

Mein Problem

Speed up my Mac mini with a Crucial M4 SSD, and extending hard disk drive space (1TB Samsung)

Meine Reparatur

Unfortunately I first did not place the second hard drive (that one where the original drive was) in the right way, so I had to tear down again although I was nearly ready to close the mini...

After doing this again (and positioning the hard drive correctly), all went as it has to be...

I needed one evening (about 2 to 3 hours).

Mein Rat

Be sure you have installed the upper hard drive correctly (screws must fit into holes!), otherwise you have to to tear down again;)

Also the power cables should be in the right place.