If you want to delete an old version (which worked perfectly well for me - I am using a 250GB external drive to backup pictures and documents), note the following: You should always delete your backup at the backup set level, and not the individual incremental backups within the backup set (see The Storage Team at Microsoft - File Cabinet Blog).
If you do so nonetheless (I've done it before ), you will have to reformat the drive and do a complete backup once again...