Maybe this can point you in the right direction. Create a script and put it in a place where you are comfortable with it being accessed from BESR. Mine is C:\DelExtBESR.bat Script example is below.
You can edit your job or create a new one. When you are on the Options page, click the "Command button" 3/4 down the page. Click the Command files folder "Browse button" Select the drive or folder etc. where the bat file you created resides. Click on the drop down arrow "Run before snapshot creation" and select the bat file. Beside that I set the timeout seconds to 2 minutes to allow the deletion of the old v2i files to take place. (This will depend on how large the v2i files are) Click OK, Set the schedule to run job...
My example is a folder on the E drive and the bat file name DelExtBESR.bat :
Script 1 is to delete everything in the folder:
rem This deletes everything inside of folder that you are backing up into "Backups"
rmdir e:\Backups /s /q
or
Script 2 is to delete everything on the external drive:
rem This deletes everything on E drive
rd e: /s /q
I hope this helps
Note:
The log file will show the backup creations and deletions but you will not see a record in the Backup History page.
I tested this on an Enterprise 2008R2 server last night. I hosed the server and restored without a problem using the off-site copy.