If iyou select standard B2d, the only way I've found to get BackupExec to acknowledge that USB drives has been swapped is to bounce the service.
If you use B2D on Removable Media, after running the wizard, backup exec sees the newly mounted drive (even though is says it can't find any removeable media)
You still need to assign each Hard Drive it's own drive letter and make sure that it is mounted on that letter every time that it is attached.