1) To properly rotate USB drives, make sure that they are configured correctly. See my article
https://www-secure.symantec.com/connect/articles/how-rotate-external-harddisks
2) BE will only span disks if the other disks in the pool are on-line and you have configured the B2D folders to span. See my article
https://www-secure.symantec.com/connect/articles/how-span-disk
Note that disk spanning does not apply to GRT backups which is what you are probably using.
3) If you are doing GRT backups, make sure that your jobs are set to overwrite and not append. If you set them to append, then the .img files may not be overwritten.
4) Check the OPP of the backups sets. Make sure that it is not set too long so that there are a lot of copies on your disk.