There are three items in the preferences:
Emulated tape drives are useful for testing and for learning how the program works. Can be set to 0, up to a maximum of 9.
Set this flag on if you are having hardware or driver problems, and then send us the log file and system profile by selecting the Send Feedback option from the Help menu.
Filmarks are used to separate the tape headers, file data, and trailers. They can either be written in overlap mode, which keeps the tape moving at speed, or not, which is much slower. Some tape drives with older (bad) firmware are not capable of overlapping filemarks, and will not write them at all unless this option is disabled.
A better solution is to update the firmware, which at present can only be done on a PC, or on your Intel Mac in Windows mode.