Problem with MAGNEX 24X CD-RW

I’ve got 2 24x CD-RW labelled MAGNEX. I’ve tried to burn them with my SAMSUNG SW-252F and my Plextor PlexWriter Premium.

SAMSUNG: max speed recognized 16x. Burning starts and proceeds without any errors. After burning, it has big trouble recognizing the disk (most trials fail).

PLEXTOR: max speed recognized 24x. Burning starts, but when lead-in is finished the speed gets down to 16x (sometimes I get an error from Windows: “disk not formatted or unknown file system”). After burning it usually has no trouble recognizing the disk.

My Philips DVD TV player has trouble reading the disk, written either with the plextor or with the samsung – usually stops reading in the middle or beyond.

I don’t know if it is a problem of my units or not. I’m thinking the CD-RWs are bad because: 1) the manufacturer is unknown (no code on the ATIP…) and 2) my drives have no trouble burning another Philips 10x CD-RW.