Has anyone else experienced this for the Xbox 360?

I am unable to install the add-on packs "Shivering Isles" and "Knights of the Nine" onto my 360. I am trying to install the packs from disc; and I have tried unsuccessfully with both the original Shivering Isles stand-alone disc and the Shivering Isles disc from the recent Oblivion 5th Anniversary Edition.

I can run the main Oblivion disc and install to my hard drive successfully. The software is version 1.2.1 which is, according to Bethesda's website, the latest version.

When I insert the add-on disc, the Dashboard doesn't show a picture for the game - it just states "play game". I click on this and the software runs, reporting "Expansion Pack Installer" and that "An update is required to install this expansion pack. Update now?". I select "Yes, update now". An "Installing" message appears for about half a second. This 'update' doesn't seem to work like updates for any other 360 game; no download (green bar) appears.

Afte this half-second update, the option "Install Expansion Pack" appears. I select this and then "Install selected" (both add-ons are automatically ticked). An "Installing" message appears but after a few seconds the message"! Expansion Pack Installer - Can't install one or more items" is reported.

Please would anyone know what is causing this error? As I say, I've tried with two different pack discs. I have successfully used the discs before; but I had deleted from the drive and now want to re-play it. I have already tried emptying the hard drive cache.

Any help wold be appreciated. Thanks!