Hello jasch
Thank you for your inquiry. It is possible the Seagate Nytro 1551 has a Power Disable Feature (PWDIS) under SATA v3.3, where if the drive receive power from the 3.3v line, the drive will power off. Since the MB993SK-B uses a 4 pin Berg connector for power, the 3.3v line does not transmit, while the 15 pin SATA power would transmit power on the 3.3v line.
There is two ways to test if this is the issue with the Seagate Nytro 1551 drives. One option is to use a Molex 4 pin to SATA 15 pin power connector converter.
The other option is to put some tape over the 1-3 pins on the 15 pin SATA power connector on the drive. Example is show here.
Please let us know if these suggestions allows usage of Seagate Nytro 1551 drive in the MB998SP-B.