SBS 2011 Backup Drive

Does it just overwrite, or generate an error and fail?  There have been a lot of issues with various backup dries over the years.  There is a list of approved and tested backup drives but generally they are 2TB drive, but that may have to do with SNS 2008 2 TB and NTFS limitations.
https://social.technet.microsoft.com/wiki/contents/articles/1780.windows-small-business-server-external-backup-drives-compatibility-list.aspx

You may also want to review the following regarding limits.  SBS still cannot backup individual volumes greater than 2 TB in size:
https://social.technet.microsoft.com/wiki/contents/articles/1919.sbs-2011-v-backup-restore-and-disaster-recovery.aspx

it overwrites , leaving no history on the drive , none of the partitions to be backed up are over 2 tb

Might the drive be low on space?  If less than 15% free space, the "auto-delete" function will kick in and delete old backups.  If it deletes an original backup and not an incremental, all files are deleted, as there is no "master" and it starts fresh.

Disk has pleanty of free space , we are getting this issue with 2 drives that are the same

What are you using as a backup program?  I don't believe that Windows Server Backup for anything before Server 2012 will work with Advanced Format Drives, which would include anything over 2T.

we are using the SBS backup program,