SSD not working internally
The problem was the internal SATA cable. After exchanging it - this is quite an easy operation, there are plenty of videos on Youtube that describe exactly how it's done - the drive booted flawlessly and is running at a negotiated link speed of 6 Gigabit.

To make sure an SSD will work with a Macbook, the Link Speed of the SATA controller should be checked in About This Mac &gt; System Report &gt; Hardware &gt; SATA/SATA Express