I get the following scsi check condition on every first access to the tape 
after a tape change:

st0(ncr0:4:0):  Check Condition on opcode 0x0
    SENSE KEY:  Unit Attention
                EOM Detected
     ASC/ASCQ:  Not Ready To Ready Transition (Medium May Have Changed)

In other words, the sequence 

$ mt offl
    <insert new tape and wait for drive to become ready>
$ mt status
mt: /dev/nrst0: Input/output error

will produce the check condition. Note that mt reports an I/O error - tar or 
dump do the same and will abort because of that. 

Which component (drive, controller or driver) are at fault and what can I do 
to correct this?

Details about the system:
i386, Pentium, running -current as of Oct 18.

SCSI Probes:
ncr0 at pci0 dev 20 function 0: ncr 53c810 fast10 scsi
ncr0: interrupting at irq 15
ncr0: minsync=25, maxsync=206, maxoffs=8, 16 dwords burst, normal dma fifo
ncr0: single-ended, open drain IRQ driver
ncr0: restart (scsi reset).
scsibus0 at ncr0: 8 targets
sd0 at scsibus0 targ 0 lun 0: <QUANTUM, XP34301, 1071> SCSI2 0/direct fixed
sd0: sd0(ncr0:0:0): 10.0 MB/s (100 ns, offset 8)
4106MB, 4076 cyl, 20 head, 103 sec, 512 bytes/sect x 8410200 sectors
sd1 at scsibus0 targ 1 lun 0: <HP, C3725S, 6039> SCSI2 0/direct fixed
sd1: sd1(ncr0:1:0): NCR quirks=0x42
sd1(ncr0:1:0): 10.0 MB/s (100 ns, offset 8)
2047MB, 3703 cyl, 9 head, 125 sec, 512 bytes/sect x 4194058 sectors
cd0 at scsibus0 targ 2 lun 0: <MATSHITA, CD-ROM CR-504, ST23> SCSI2 5/cdrom removable
probe(ncr0:2:1): 5.0 MB/s (200 ns, offset 8)
st0 at scsibus0 targ 4 lun 0: <EXABYTE, EXB-8500SMBANXH1, 0458> SCSI2 1/sequential removable
st0: st0(ncr0:4:0): 4.0 MB/s (250 ns, offset 8)

-- 
Andreas Wrede              Planix, Inc.
andreas@planix.com         Networking, System Administration, Consulting
http://www.planix.com      Toronto, Ontario, Canada