how "reformat" DLT IV tape from 20/40 to 40/80?

[Palle Girgensohn]

Hi!

Just got a new taper, an HP DLT VS80. We previously had an Adic, 20/40. The 
use the same type of tapes, DLT IV. Now, I want to keep using some of the 
old tapes, but the HP won't write to them, and I believe that is because 
they are just 20/40 GB, not 40/80 GB. I found some info saying that the 
first time a tape is used, the taper writes info about the density (or sets 
the density somehow).

I use FreeBSD, so I expect mt(1) should somehow be able to perform the 
trick. Some tapers seem to have a button for switching density, but this 
machine doesn't.

Odd thing is, "mt stat" always returns 0x41 density, which is 40 GB. Both 
for new and old tapes:

Mode      Density              Blocksize      bpi      Compression
Current:  0x41:DLTapeIV(40GB)    variable       98250    IDRC
---------available modes---------
0:        0x41:DLTapeIV(40GB)    variable       98250    IDRC
1:        0x41:DLTapeIV(40GB)    variable       98250    IDRC
2:        0x41:DLTapeIV(40GB)    variable       98250    IDRC
3:        0x41:DLTapeIV(40GB)    variable       98250    IDRC
---------------------------------
Current Driver State: at rest.
---------------------------------
File Number: 0  Record Number: 0        Residual Count 0


Any ideas how to fix this?

Thanks!

Palle