problem mount hard disk using loopback device

Hello, I have one question to ask:

When I try to use Knoppix 5.0 liveCD, I can mount any of my partitoin using loopback device, that's say:

mount -t vfat -o offset=`expr 234567821 "*" 512` /dev/hda /mnt/hda5 as an example. But in my own Debian system . I can't do that, error message is:

/mount: wrong fs type, bad option, bad superblock on /dev/loop0,
or too many mounted file systems
(could this be the IDE device where you in fact use
ide-scsi so that sr0 or sda or so is needed?)/

and dmesg:
/loop: loaded (max 8 devices)
FAT: bogus number of FAT structure
VFS: Can't find a valid FAT filesystem on dev loop0./

you may ask me why I do this but don't use /dev/hda5 directly. Because in my harddisk, there is a host protected area(hpa) which is a hidden partition to store some info. So, actually I don't have /dev/hda*, but I can use knoppix to mount it through its offset, because i know its partition's offset. Now. I just want to know why my debian system can't.

BTW, older knoppix livecd can't mount and give me the same error info. Is it because new kernel? But i try to use 2.6.18 and also can't do it.

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