The tomtom has a SD card reader/writer built in the device (it writes and updates files such as mapsettings.cfg, itinerary and favourites) and can be used as an SD reader/writer when connected via USB so it is perfectly feasible that while the SD card was in the device either while updating the BL or after while rebooting that it malfunctioned and damaged the card causing the unallocated partition. Which is exactly what happened. The reason the SD card wouldn't work was because of the unallocated partition, without it, it works perfectly. That partition was not there before I updated the BL I had checked and if so the BL update would have failed as the device would have been unable to read the SD card. It was there immediately after and was thus not recognised in tomtom.
The moment it rebooted after the BL update it did not work and I didn't put in the PC in between so it couldn't have affected it at all. I've been updating Roms and building programming and repairing systems for over 25 years so perfectly understand about formatting an SD card but thank you for your advice.
I am aware that under normal circumstances it cannot create partitions intentionally but as it is a writer and the card wasn't locked it could have malfunctioned and caused the problem causing the damage to the card. Thanks for reply but your assumption is incorrect because as i said the fault occurred when the card was not in the PC, the sequence of events i described show that.
Hopefully if other users encounter the same issue with a faulty SD reader/writer in their tomtom damaging their SD card they can fix it in the manner I have.
Thank you both again for your comments and hopefully it may help others if needed.