Yes, sd0 was a typo ...

I typed:
modprobe st
but again:
[root@asrLXsrv ~]# mt -f /dev/st0 status
/dev/st0: No such file or directory
even if:

[root@asrLXsrv ~]# lsmod
Module Size Used by
st 34781 0
parport_pc 24705 1
lp 11565 0
parport 41737 2 parport_pc,lp
autofs4 24005 0
i2c_dev 10433 0
i2c_core 22081 1 i2c_dev
sunrpc 160421 1
ipt_REJECT 6465 0
ipt_state 1857 0
ip_conntrack 40693 1 ipt_state
ip_tables 16193 2 ipt_REJECT,ipt_state
vfat 14785 1
fat 45409 1 vfat
dm_mod 54741 0
md5 4033 1
ipv6 232577 8
uhci_hcd 31449 0
8139too 26305 0
mii 4673 1 8139too
floppy 58609 0
ext3 116809 2
jbd 74969 1 ext3
raid0 7617 1
aic7xxx 150809 0
sd_mod 16961 0
scsi_mod 118417 3 st,aic7xxx,sd_mod

In /dev I've got a lot of "tapXX" like "tap0" "tap1" and so on but I don't think it stands for "tape" ...
Thank you again.

Is the tape drive recognized by the SCSI controller during its BIOS boot scan?

Here again! :)

Well, since the tape drive wasn't recognized at boot time (thank for the hint) I changed the SCSI controller.
Then, here what I get without a tape inserted :

[root@asrLXsrv sbin]# mt -f /dev/st0 status
SCSI 2 tape drive:
File number=-1, block number=-1, partition=0.
Tape block size 0 bytes. Density code 0x0 (default).
Soft error count since last status=0
General status bits on (50000):
DR_OPEN IM_REP_EN

but if I try to insert a tape:

[root@asrLXsrv sbin]# mt -f /dev/st0 status
/dev/st0: Input/output error

I also tried to use KDAT but it says there's not a tape in /dev/st0 ...

Thank you again in advance for any hint.

PS: I've got also a Surestore DAT40 ... do you know if this is better "recognized" by Linux?

I've just tried another backup program but nothing changed ... anyone could help me please, o tell me where I could find tutorial o documentation on tape streamer and Linux? Thank you.

I've no idea why it didn't work for darkmage1975, but I've got a SureStore T20 connected to an adaptec SCSI card and it works fine with 2.6.27 kernel. For reference:
* I've disabled the SCSI host adaptor BIOS because I don't boot from the card.
* tar and mt communicate with the drive fine.
* tar runs much quicker with a larger blocking size specified (-b128) which also seems to make the drive more reliable.
On an Athlon 2200XP CPU you can use software compression without slowing the tape down by adding z to the tar options (if your backup isn't going to need multiple tapes - you can't compress multi-volume archives).