You imported from Outlook Express. That should be OK, but it wasn't. So you try copying, this way:
1. In Outlook Express, make a (OE) folder with the same name as the missing one. Close Outlook Express.
2. Go into Windows Explorer and find the .dbx-file you just created (it's in the 'archive folder')
3. Find that same .dbx-file on the external hard drive.
4. Copy the file from the external hard drive over the file you just created.
5. Open Outlook Express. You'll find the (OE) folder you just created filled with the contents from your backup.