We have a msaccess(2003) database running on several laptops.
You will have to explain that, and the rest of it, better.
You have one split db with the tables in one place and every pc has it's own front end connected to that backend?
Each pc has it's own complete db? I hope not.
How can I check the right Drive-letter where the external drive is connected?
Why? Are you trying to run these backups programmatically?
If you answered yes - that each pc has it's own complete db and you are trying to sync them, then you might want to think about that before worrying about how to know what drive letter a pc uses when connected to the usb. Even if you saved this info in a table and matched it to the pc id, you couldn't even rely on it being constant. It would likely change if a user had a remote dvd connected one time but not another.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.