mkfs.minix was not compiled with minix v2 support.
Run "mkfs.minix -v /dev/sdb6" (well, probably wiser not to), and
you will receive "mkfs.minix: not compiled with minix v2 support".
The mkfs.minix from debian testing can make v2 filesystems which
the (well, my 2.5.6) kernel is happy to mount.
It appears that fsck.minix doesn't understand v2 either. Given a
v2 filesystem, it complains "fsck.minix: bad magic number in super-block"
Can you try the package in Skipjack or rawhide?
As far as I can tell, HAVE_MINIX2 should be working fine there.
Sorry for the delay, it does seem fine in Skipjack.
Thanks a lot.