Hi
I want to switch to K12LTSP - FC5 while being able to boot my current FC5 installation until I am satisfied with the new installation.

The plan:
I connect a second (new) SATA drive. Disconnect the old one and
install K12LTSP-FC5 from scratch.
Reconnect old drive and mount it readonly on say /mnt/old
so that I can copy over user data and refer to old configuration.
I am working with 2 HDDs to be sure I won't loose data by
(my) mistake.

I wonder if this is going to work. I have never before explicitly
mounted a HDD that uses LVM.

Why did you disconnect the drive? You shouldn't have to disconnect anything to dual-boot. In fact, I would think it would create more problems.

Disconnect just to be sure I do not affect the old installation in any way while installing the new system. That should be foolproof.

My doubts are about mounting on FC5 a second disk also containing a complete FC5 installation with LVM.

Ok, now you have two discs with two boot loaders. How do you expect the second OS to be an option? The way you have it set up, you have to physically swap the drives and possibly change the jumpers EACH time you want to boot to the other.

In the BIOS I can specify from which disk to boot.
However when I boot from the new installation I would also like to
mount the old disk readonly to say /mnt/old.
Once I am satisfied with the new installation and have copied everything
useful then I remove the old disk.

$mount -r <device-file-name> <mount-point>

I think I found the answer:
http://www.tldp.org/HOWTO/LVM-HOWTO/...gtonewsys.html

Have not yet tried it!

Back to square one. I am booting FC5 from disk /dev/sda.
I have a second disk with a full FC5 installation connected and it
shows as /dev/sdb

Mount as suggested above does not do it:
mount -r -t ext3 /dev/sdb2 /mnt/old
mount: wrong fs type, bad option, bad superblock on /dev/sdb2,

http://www.tldp.org/HOWTO/LVM-HOWTO/...gtonewsys.html
also does not help. Firstly I can not unmount it because it was created by the
FC5 default installation and contains most of the running system. So unmount reports that it is busy. So I cannot mark the volume group inactive. This is the first part of the recipemovevgtonewsys.html
So I simply connect the disk to the new system.
My second disk shows up as /dev/sdb2
but none of the commands pvscan, vgscan and lvscan reports that disk or the volumes on it.