[EDIT 1 day later: Maybe Sam mistyped "Assuming" . I dont think it was intended as a troll reply. ]
If you can "see" the 400 gigabytes (for example, about 200 files, most around 2 gigabytes in size) then everything went OK - just do NOT forget to copy your backup program, just in case. The files will usually be numbered in one way or the other.
If you only see that you have 400 gigabytes less on the external HDD, but dont see any new files, something went wrong :( Quite hard to tell what actually went wrong if its like that but my money would be on disconnecting the HDD before Windows (or what are you using? ) said it was OK to do so. You should CHKDSK the drive if it was that, remove any .CHK files, and retry the backup.
OTOH, it is usually better to exempt media files like VOB or AVI from the backup and just copy them to the external HDD.
Hope that helps