after few hundred megabytes wrote to tape i got
st0(esiop0:0:0:0): unhandled message 0x3
st0(esiop0:0:0:0): unhandled message 0x3
st0(esiop0:0:0:0): our reject message was rejected
esiop0: scsi bus reset
st0(esiop0:0:0:0): command with tag id -1 reset

and dump stops


is it:

1) not clean drive
2) bad cabling
3) NetBSD error (-current)