SCSI TK50 and writing tape

My attempts to boot the MicroVax continue: After much digging around at the house, I finally found another TK50 cartridge. This one is in good condition. I cleaned my SCSI TK50 drive, and mounted the tape. It read fine, I was able to dd the contents (one ~70mb VMS data file, with header and trailer) off the tape and onto my PC. So far so good. I erased the tape, and tried to write the NetBSD boot.fs to it. dd if=boot.fs of=/dev/nst0 bs=512 And the drive whirred to life, shoeshining tape back and forth,but actually making progress through the media (it was accumulating on the take-up reel, anyway). So I let it run. Eventually it failed with an I/O error at a point where most of the tape was out of the cartridge. It took over an hour to get there, and only made it through one length of tape (no track change/reverse) Rewind, erase tape again. So, I tried just plain 'dd if=boot.fs of=/dev/nst0' without specifying the block size (although from what I've read, NetBSD wants a block size of 512 to boot) That shoeshined like crazy too, so I stopped it. Try again, with a block size of 16k (the same block size I used to read in the tape). Still no go, it still shoeshines, but not quite as much (barely). What do I need to do here? How long should it take to write two and a half megabytes of data to tape? Thanks! -Ian