Okay. Thanks for the comments.
Incidentally, I received the 10 MD data disks that I ordered from MF for $9.99, just today. As many of you have commented, they shipped TDK disks instead of Sony. Since I've only used TDK disks over the past 2 years and since they I haven't had any trouble with them (knock on wood again), I'm going to keep them.
By the way, I feel no affinity to TDK. They just happen to be the brand that my local Guitar Center has stocked. As Stan Williams has implied, since they seem to working reliably for me, I shouldn't worry about using them. I was just curious why some of you were anxious to return the TDK's for Sony branded disks.
As for Sony, I agree they have developed a lot of great concepts and inventions. Oddly, I have had the most trouble with Sony branded electronics over the years, including stereo equipment, televisions and the MDM-4X 4 track mini disk recorder. I have never questioned Sony's innovations but due to my bad luck with their products, I've always questioned their quality.
Anyway, that's my own experience and opinion. Thanks again for the information.