TomTom Home probably loaded a firmware upgrade to your SD card at some point, and you've been booting off of that ever since. If there is a bootable system on your SD card, your 720 will use that before it ever looks at the memory on your unit.
OK - you can function as you do now, but it would be a good idea if you had your operating software in internal memory. Right now, it appears that there is something there, but that it cannot work.
Might wait for dhn to add his information to this before you jump into this job, but here is what I'm thinking (use THIS procedure for both internal and SD backup ->
https://www.tomtomforums.com/genera...4-how-backup-contents-your-unit-computer.html)
Copy the entire contents of your internal memory to a folder on your PC.
Copy the entire contents of your SD card to another folder on your PC.
Copy the entire contents of your SD card to your internal memory.
All in THAT order! Do NOT start with step 3!
That will assure that we have backups of everything, and will copy what seems to be working for you on your SD to your internal so you can remove the SD card.
Also - depending upon how long you've owned your 720, there was a time when it could NOT manage more than a 2GB card correctly. It would look like it was OK, but it was not. Your unit firmware must be at least 8.302 before a 4GB card can be used correctly. Since we don't know how long ago "all these years" might be, can you also verify the current firmware level of your 720 for us?