I have a multiboot system which has 2 versions of Windows and 2 versions of Ubuntu. I use GRUB to boot. There are 2 SATA disks and 1 PATA and until 2 days ago 1 DVD burner. Everything worked fine. The GRUB MBR is on the PATA drive which is the BIOS boot device.

Then I installed an additional DVD reader.

Now GRUB gives error 25 when I try to boot either Windows or Ubuntu from the second SATA drive (hd2 for GRUB sdc for Ubuntu). However if I boot the version of Ubuntu on hd0 then not only does that work but both partitions on sdc are readable.

The DVD readers both work i.e. I can read DVDs on them.

If I unplug the power from the DVD reader then things go back to normal.

There are 2 SATA ports and 2 IDE ports on the motherboard. Initially the PATA disk and the DVD burner were slave and master (in that order) on IDE 1. Now I've put the disk as Master on its own on IDE 1 and the 2 DVDs on IDE2 as Master and Slave.

Having trawled the web for something similar I haven't found anything. Does anybody have a clue what's going on?

Yes. It's all about what GRUB thinks the drive numbers are. I think that mixing SATA and PATA might muddy the water, but I'm not clear on exactly how.

Suppose the second SATA was being seen by GRUB as hd3. Adding the DVD to the PATA chain likely changes that.

You can see GRUB's version of reality by starting the GRUB shell and entering the command "root (hdX,Y)" X = 1, 2 , 3...etc.) It will respond with what is on each drive/partition)

Once you know how GRUB is seeing the drives, you can edit /boot/grub/menu.lst to match.

I think you confuse yourself and then proceed to confuse Grub.

By your own admission the Pata was a slave and the DVD was a master in the first IDE channel and would be known to Linux as hdb and hda. In other word the Pata is hdb.

In the new arrangement a second IDE channel is used for the two CD/DVD drives and so they would be known to Linux as hdc and hdd.

Moving your Pata to the Master position makes it a hda now. So Grub is still trying hdb which is no long available. So Grub reports Error 25 "Disk read error".

Your system in the Pata will not boot until you edit the hdb to hda in /boot/grub/menu.lst and /etc/fstab. Newer Ubuntu uses by-uuid method to identify the partition and this could have worked. You appear to be using an older version.

I've tried what you said, in terms of getting GRUB to tell me what drives it has detected either by using root (hd TAB or geometry (hd TAB
Both commands tell me that GRUB has found hd0 hd1 hd2 which are sda sdb sdc for Ubuntu.

As you rightly point out the latest version of Ubuntu does use uuid. Since that version is on hd0 it doesn't help me to prove much.

I have now unplugged the power on the DVD burner which is upposed to be the master on the IDE2 chain. Everything boots fine. i.e. I can boot from hd2.

I'm now wondering if the DVD burner hasn't got a bus problem but why that should affect one of the SATA disks is beyond me.

Just to remind you hd0 is PATA on IDE0, hd1 is SATA1, hd2 is SATA2

So I still have the problem.

You can move Linux around, master or slave, Pata or Sata no problem.

Just alter that Linux's

(1) /boot/grub/menu.lst
(2) /boot/grub/device.map
(3) /etc/fstab

Notes

(a) If you use /boot in a separate partition drop "/boot" in (1) and (2)
(b) If the Linux belongs to the Red Hat family, use grub.conf instead of menu.lst
(c) You need to update the boot loader that controls the MBR if the Linux being moved is not the system shouldered with the multi-booting function.
(d) If the Linux uses Lilo you can amend /etc/lilo.conf instead of (1) and (2) but you need to revalidated Lilo with the command "lilo -b /dev/hd??".