Tape media

Hi, so I sorted through my last weekend's bait, hooked up my new HSZ40 (with 50 GB in 4 StorageWorks boxes plus all sorts of tape drives, TSZ07, TLZ07, TZ68, and ~88) to my PC for FreeBSD. It's so much fun to read a 9-track reel from a FreeBSD PC (just too bad it doesn't have a transparent top cover to show off the reels). I have bulk-erased a bunch of TK50 tapes in the hopes that they would work as DLT III and IV tapes as well, but these drives still went into write-protect mode. Does anyone here know details about the DLT media? I mean it is a well-known fact (and I proved it to myself many times) that CompacTape I works just fine as CompacTape II. So why not as III and IV? Is it really the tape or might it be some notches on the cassette that are used to code the kind of tape? The problem is I didn't get any tape media for my nice DLT tape drives and this stuff is so darn expensive if bought new. Any ideas? regards, -Gunther