My mainboard is an Asus P5LD2, with two ata-controllers. The first ata-controller is single-channel for two drives, and on this controller is sda.
The second ata-controller is dual-channel with sdb on it.

I am running Kubuntu 7.04 (FF) with the Kde desktop environment.

Several times now I have had a problem with /etc/mtab.
I have two harddisks, sda and sdb. In the normal setting /etc/mtab looks like this:

/dev/sda1 / ext3 rw,errors=remount-ro 0 0
/dev/sdb1 /music reiserfs rw,noexec,nosuid,nodev 0 0

In the faulty situation sda and sdb are swapped, sda becomes sdb and reverse, in /etc/mtab:

/dev/sdb1 / ext3 rw,errors=remount-ro 0 0
/dev/sda1 /music reiserfs rw,noexec,nosuid,nodev 0 0

The result is that the root directory / becomes a subdirectory from /music, and if I try to boot this system things go wrong.
When this occurred for the first time I didn't know why my system didn't boot properly.
So I started with the install-cdrom and selected the repair option.
Fortunately I was able to edit mtab and after that my system booted properly.

Both harddisks are set as primary disk, but that should not confuse the system.
This is puzzling me, and I'd like to get hints that can answer the question why this happens.

That's very strange, usually you shouldn't even edit mtab. I suggest you check /etc/fstab for screwed up entries. fstab tells the system which partition should be mounted under which directory at boot up time. mtab basically says which partition is currently mounted under which directory, and therefore shouldn't really be edited.

-A

I believe it is a kernel problem. I have a similar problem with Fedora.

Even though each disk is not in the other's fstab entry, the kernel updates go to the wrong /boot

I infer that you are using the ide/pata and not the SATA. You say one drive on the single-channel, and one on the 2-channel---those are both pata.

If this is true, then why not put them both on one controller? (Either one, since 1 pata channel will handle two drives)
Be sure to set the jumpers so one is primary and one is slave.

Even with them on separate controllers, having both jumpered as primary does not sound like a good plan.

From the spec for the board:

I do not want the / partition in a raid.
I use the single-channel controller for my system disk and the dvd-rw.
In the near future the sata controller as well as the dual pata controller will be used for raid.
Until then I'll configure the sdb disk as a slave!