Are you appending Diff data to a BKF file and never overwriting?

Normal setup would be do ful backup followed by diffs, then after next full backup allow the older diffs to be overwritten, This is controlled by getting your overwrite and append settings correct in the media set used by the differential jobs. - Would recommedn the full job uses a different media set with stonger retention settings.

Does it means that the diff gets overwite after the append period is over?

there is no way deleting the entire diff data for month using the backup exec, however i would suggest you to create a new media set
with overwrite period of 30 days and appened period infinite, than move all your b2d in the backup exec to the new media set created......once done that click on tools | options | media management and select the option overwrite recyclable media before overwriting scratch than click on ok.....than edit the diff job and select the new media set created and run the job

by following the steps above it will not overwrite the entire data however will overwrite the oldest backup set and you would be able to protrct your differentials for a month.....

this should go ahead and automate the process where in you dont have manually delete the old backup sets as new will overwrite them,

reason i suggessted you to use the option overwrite recyclable before overwriting scratch cause with B2D configuration if u have the option selected backup exec does not delete the backupsets until disk is full or meets the threshold.......hence using second option forces backup exec to overwrie the oldest backup set and B2D folder size does not further.

let me know if this answers your questions, if yes please mark it as solution