[cctalk] m4 model 9914 tape drive and 800dpi 9track tapes

I have found that a modernish Dell PC (win 7 era) when attached to a m4 9914 tape drive has issues when attempting to read 800dpi 9track tapes. I/O errors that seem to be centered around variable block size microcode of the modern scsi controller (?) I've spent a number of hours on it and I could not even pull raw data off an 800dpi tape. 1600 no problem. works fine. is there a tweener box or scsi controller that will work in this situation? what do other people do? Bill