Here are some details on what I have done, and the behaviour that I get. If you need more info/details, please let me know the specifics. Otherwise, can someone tell me what I am missing?

In the amanda.conf:
===========================================================
...
runtapes 1              # number of tapes to be used in a single run of amdump
tapedev "rait:{file:/backups/current1,tape:/dev/rmt/0bn}"       # mirrored backups
rawtapedev "/dev/null"  # the raw device to be used (ftape only)

tapetype BIGDAT         # what kind of tape it is (see tapetypes below)
labelstr "^flxmth[0-9][0-9]*$"  # label constraint regex: all tapes must match
...
define tapetype BIGDAT {
    comment "BIGDAT tape drives"
    length 20000 mbytes         # these numbers are not accurate
    filemark 100 kbytes         # but you get the idea
    speed 100 kbytes
}
...

The rest of the config should be standard/irrelevant.
=============================================================
Here is what happens when I try to run amlabel:

amanda@matrix $ pwd
/backups/current1
amanda@matrix $ ls -la
total 18
drwxr-xr-x   7 amanda   amandagrp    512 Jan 30 14:58 .
drwxr-xr-x  12 root     sysadmin     512 Jan 27 17:38 ..
-rw-r--r--   1 amanda   amandagrp      0 Jan 30 14:58 am.log
lrwxrwxrwx   1 amanda   amandagrp      9 Jan 30 14:58 data -> flxmth005
drwxr-xr-x   2 amanda   amandagrp    512 Jan 28 09:43 flxmth001
drwxr-xr-x   2 amanda   amandagrp    512 Jan 28 12:11 flxmth002
drwxr-xr-x   2 amanda   amandagrp    512 Jan 30 12:39 flxmth003
drwxr-xr-x   2 amanda   amandagrp    512 Jan 30 14:48 flxmth004
drwxr-xr-x   2 amanda   amandagrp    512 Jan 30 14:58 flxmth005
-rw-------   1 amanda   amandagrp     11 Jan 30 14:53 info
amanda@matrix $ ls -l flxmth005
total 0
amanda@matrix $ more info
position 1
amanda@matrix $ ammt -t file:/backups/current1 rewind
amanda@matrix $ mt -t /dev/rmt/0bn stat
Vendor 'SEAGATE ' Product 'DAT    9SP40-00' tape drive:
   sense key(0x0)= No Additional Sense   residual= 0   retries= 0
   file no= 0   block no= 64
amanda@matrix $ mt -t /dev/rmt/0bn rewind
amanda@matrix $ amlabel -f flxmth flxmth005
rewinding, reading label, reading label: Error 0
rewinding, writing label flxmth005, checking label
amlabel: reading label: Error 0
amanda@matrix $ mt -t /dev/rmt/0bn stat
Vendor 'SEAGATE ' Product 'DAT    9SP40-00' tape drive:
   sense key(0x0)= No Additional Sense   residual= 0   retries= 0
   file no= 0   block no= 64
amanda@matrix $ ls -l flxmth005
total 132
-rw-------   1 amanda   amandagrp     10 Jan 30 15:01 00000-unknown
-rw-------   1 amanda   amandagrp  32768 Jan 30 15:01 00000.unknown
-rw-------   1 amanda   amandagrp     10 Jan 30 15:01 00001-unknown
-rw-------   1 amanda   amandagrp  32768 Jan 30 15:01 00001.unknown
amanda@matrix $ more info
position 2
position 2
====================================================================
This already seems wrong. Here is what happens when you then try to run an amdump:

amanda@matrix $ mt -t /dev/rmt/0bn rewind
amanda@matrix $ ammt -t file:/backups/current1 rewind
amanda@matrix $ amdump flxmth doom c0t0d0s3
amanda@matrix $ ls -l flxmth005
total 70340
-rw-------   1 amanda   amandagrp     10 Jan 30 15:01 00000-unknown
-rw-------   1 amanda   amandagrp  32768 Jan 30 15:01 00000.unknown
-rw-------   1 amanda   amandagrp     13 Jan 30 15:04 00001-unknown
-rw-------   1 amanda   amandagrp35946496 Jan 30 15:04 00001.unknown
amanda@matrix $ mt -t /dev/rmt/0bn stat
Vendor 'SEAGATE ' Product 'DAT    9SP40-00' tape drive:
   sense key(0x0)= No Additional Sense   residual= 0   retries= 0
   file no= 3   block no= 0
=====================================================================


Cheers,
Milos