Yes, there must have been a bad connection. I am still running at SATA III, without any fall-back until now. So I guess my problem is solved.
I hope the threat starter was lucky too.

Hmmm. I based my opinion on my experience with my PC. The Samsung was slower then the SKHynix. Your post activated my mind. In my HP the Samsung SSD is mounted in a "converter" to mount it in a 3.5"...

Rebooted some time ago now. Did a performance test with the samsung software on de samsung SSD. Immediately after the test, SATA speed changed from III to II. I did the speed test again. There is...

I have the same problem with a Samsung 850 Pro SSD (250GB) in Windows 7 x64.
SkHynix SSD continuously connected with SATA III.
WD HD continuously connected with SATA III.
HP DVD continously...