I changed my approach instead of copy/paste I used drag and drop. This works (but takes a lot of time!), but I cannot understand why all the clips I import are double in size (to be exact: the original clip is for instance 100 MB in my 'Mother' SD project the imported clip in my HD project is at least 200 MB)
This is killing in respect to my disk space.
I asked a whiskid on my Mac club; he came with a clear answer: after importing the clips the HD project is making extra black inserts rigth and left (which takes space) because it was 3:4 movie and there is upscaling to fit into the HD project which is making the clip (data) of course also larger. (also he told me that the decoding in this way is probably not very efficient and makes the file also bigger). I am going to buy a huge external HD to make the transfer possible.