The burner was connected to GIGABYTE GA-890FXA-UD5 rev 2.1 motherboard. This board has 2 SATA controllers:
1 AMD South Bridge: 6 x SATA 6Gb/s connectors supporting up to 6 SATA 6Gb/s devices
1 Gigabyte SATA2 controller: 2 x SATA 3Gb/s connectors supporting up to 2 SATA 3Gb/s devices
I connected the disk for OS and Plextor to this Gigabyte controller in AHCI mode
Plextor worked fine during installation of WIN7-64bit.
After installation of OS I tried to burn a disk and then it appeared the disk was recognized only as a CDROM-device not capable of erasing/burning disks. This was also revealed by the plexutilities self test where all results were negative. (Upgrading firmware from 1.03 to 1.05 did not help either)
In order to finish the installation I bought another PX-L890SA and to my surprise it showed same behavior. I could not believe 2 burners were in error so I connected the burner to the Southbridge controller (also AHCI-mode) and to sum it up: All went fine: Plexutilities selftest all OK, and burning CD-RW without problem.
Hopefully this can help you in future situations,