NetBSD TK70 question

I'm not sure where I should start asking, so I'm starting here ;-) I have a problem reading TK70 (and probably TK50) tapes in NetBSD 3.0 on a MicroVAX II. There is absolutely no way of reading a single tape block with a simple read(). All I get is mt0: unknown opcode 0x80 status 0xc01 ignored on the console, and then the driver hangs. The output is generated in /usr/src/sys/dev/mscp/mscp.c It is my impression that the code has *never* been tested on real hardware after all that years. BTW the TK70 is working fine otherwise (e.g. I can boot the MVII diagnostic tape). Now for something strange: the same procedure works in SimH (with the same system installation and kernel). So apparently SimH has a "bug", too. It doesn't behave like the real device. Background of the story: I want to image TK70 tapes as TAP files. Has anyone ever encountered the same behaviour? Christian