Go 300 Red Cross on SD problem

When I power on the Tomtom the SD card is shown with a big red cross. This happened after I tried to install some new voice files and filled up the SD card. Aha, I thought! I'll just restore from the backup I took before I started meddling with it. The restore goes through OK, but the problem remains. I've even tried a different SD card with the same result. I'm sure the SD cards are fine as I can read and write to them through explorer when it's attached to my PC. I've logged a question with Tomtom support, but no reply is forthcoming.
Anyone have any ideas?

Just got a response from tomtom and it worked! they said:
1: Make a full back up of your SD Card.

2: Delete your SD Card.

3: Install the latest 5.440 update to your Device, you will need to log in to the Tomtom website, select on the support page, select on the 1st question ' Update March 2006 tomtom GO 5.440', follow the instructions on how to download and install the Update on to your Device

4: Re-install your Backup. The only Folders that you will need to re-install are the Voice, Map and Zip Folders.