NT& backing up to tape

'I've read IIUG's FAQ and all the .PDFs on Informix's web site, but can't find the answer to this problem (although I've learned a lot about backup procedures). My NT 4.0 server uses a Colorado Jumbo 350 for backing up logical log files. I *can* backup files using NT backup, so I know the drive works. Informix backup reports that the block size is not supported by the device (but never appears to access the device). I have tried 4, 8, 16, and 512K (the apparent block size the drive uses on the tapes, according to the specs) block sizes, in the onconfig.std file. Does anyone know the trick to making Online Workgroup Server 7.2 backup on NT? Sincere Thanks, D