Cant write filemarks on DDS-4 Tapes

[pcasidy at casidy.com]

Hello

I currently face a strange problems.

I use several type of tapes; from 1Gb to 20Gb tapes.
I can read and write from of them but DDS-4 (20Gb)!

I can read DDS-4 btu cannot write on them.

When I try to write on a DDS-4 I encounter an error:
[gueway] ~# tar c /
tar: Removing leading `/' from member names
tar: /dev/sa0: Wrote only 0 of 10240 bytes
tar: Error is not recoverable: exiting now

and /var/log/messages says:
Jun  5 12:28:09 gueway /kernel: (sa0:sym0:0:5:0): WRITE FILEMARKS. CDB: 10 0 0 0 1 0 
Jun  5 12:28:09 gueway /kernel: (sa0:sym0:0:5:0): MEDIUM ERROR info:1 asc:c,0
Jun  5 12:28:09 gueway /kernel: (sa0:sym0:0:5:0): Write error
Jun  5 12:28:09 gueway /kernel: (sa0:sym0:0:5:0): failed to write terminating filemark(s)

every time a filemark has to be written, I have the same error.

For information, mt status gives:
[gueway] ~# mt status
Mode      Density              Blocksize      bpi      Compression
Current:  0x26:DDS-4           variable       97000    DCLZ
---------available modes---------
0:        0x26:DDS-4           variable       97000    DCLZ
1:        0x26:DDS-4           variable       97000    DCLZ
2:        0x26:DDS-4           variable       97000    DCLZ
3:        0x26:DDS-4           variable       97000    DCLZ
---------------------------------
Current Driver State: at rest.
---------------------------------
File Number: 0  Record Number: 0        Residual Count 0

Changing comp to off, or blocksize to 1024 does not help.

I do not understand why i can write on DDS-2 or DDS-3 but not on DDS-4
(i used to several months ago).

I need your help ;)

Thanks

Phil.