I recently returned from a few days away, and when I came
to plug in my iPod the iMac told me that I had "inserted a
disc containing no volumes that OS X can read". The iPod still charged but wouldn't show up in iTunes or on the desktop. It then
transpired that the same thing happened if I inserted a
blank CD in the external CD burner. I tried re-installing
OS X from the discs but can't as I get the same message
as before, and can't start the installation. Any suggestions
would be appreciated.

rhinocrat

Okay. It sounds like you have two separate problems
here. Let's try tackling them one at a time.

First, your iPod problem. If you have your music already
on your computer, I would recommend erasing your iPod
and putting the iPod software back on the unit. To do this
make sure you have the latest OS and iPod Software
Updated available to you by first connecting to the
internet then clicking the Apple in the top left corner and
selecting "Software Update..." in that menu. If you have an
older OS (OS X 10.2 or earlier) you can find Software
Update in your System Preferences. After you make sure
you have these two things, connect your iPod to your
computer again then navigate to Macintosh HD-
>Applications->Utilities->iPod Software. Within this
folder you should have at least one version of the iPod
Software Updater. Lauch the latest (they have dates
associated with them) version you have and select the
"Restore" button once the iPod information appears. This
will erase the entire iPod and install the iPod software
again. If this utility does not see anything connected you
either have some OS problems (which I deal with as part
two of your answer) or the iPod or iPod cable have become
defective. If your unit is still under warranty you can go to
http://depot.info.apple.com/ipod/ to request a box to
ship your iPod into the Apple Service Center.

Second, your OS/external drive problem. First, I would
create a new user by going to System Preferences then
selecting the Users option normally in the bottom left
corner. There click the '+' button below the listed user(s)
to input information for a new user. Log in as that user
and see if the external optical drive will or will not read
disks. If it does read disks you know there's something
wrong with your users folder on the other account and will
take some different steps (normall just deleting of the file
in Macintosh HD->Users->My User->Library-
>Preferences associated with the external drive) but be
careful in here. If the drive does not work that means the
OS itself is at fault here and you need to reinstall your OS.
This is not as hard as it seems and there is an option that
will reinstall your entire OS while leaving your Users folder
alone. Find your OS disks and place the first one in your
optical drive. Restart the computer and hold the 'C' key
down. This will start the computer from the CD rather
than the internal hard drive. Follow the screen prompts
until you reach a screen that has you select your hard
drive as your destination volume you would like to install
to. There should be an "options" button in the lower left
corner. Click that. There will be three options and you
want to select "Archive and Install" as well as the sub box
that says "Preserve User Settings and Accounts". Keep
going from there and it will reinstall your OS.

Good luck!