You did all the correct things for backup and restore. Make sure to copy the DBX back from USB Flash Drive to hard drive, make sure the files are not write protected. Right click on each file and select Properties. Uncheck Read Only. If no joy, do you have any backup DBX files to use? If YES, then try to restore files from backup. These seemed to be corrupted beyond repair. If NO, try a better repair tool - OE-Mail Recovery - free trial version. http://www.oemailrecovery.com/dbx-extract.html
