iris cartridge tape drives

[Mark A Allyn]

I have an Iris 3130 workstation that has a 1/4 inch cartridge tape drive
as /dev/rmt1. I have been doing my backups using tar which I assume
uses this device with a default blocking factor of 400. I have
a few questions about this:

1. We use 600 foot tape cartridges which have an alleged capacity
   of 60 mb. Can anyone varify this?

2. I have a problem right now where I have tried to tar a directory
   tree that I know is about 50 mb by using the du command on this
   directory tree. The tar utility will come back with a message
   'tape error or end of tape - insert new tape to continue'. 
   My question is if the 600 foot tape is supposed to be able to
   hold 60 mb, then why is this doing this? I have tried several
   tapes with the same results. I have also rebooted the system
   with the same results.

3. I have been told that if tar does require another tape to 
   complete its job, that the system may crash and that the
   tape drive goes into a funny state. Is this true, and why, and
   what could I do to alleviate this problem. Is this a problem with
   the drive itself, the software device driver not 'cleaning up' after
   the end of tape error, or what?

Thanks in advance!
Mark Allyn
Boeing Aerospace
uw-beaver!ssc-vax!ssc-bee!maa