Removable Backup to Disc 'Not Appendable'

[cestmoi]

 I'm backing up to external SATA drives using BE 9.1. Each drive is 1.5TB size but I have configured the backup file to be 1000GB size. We run Differential backup (which is around 70GB) from MON-THU on the same drive. Overwrite protection is none and Appendable is Infinite...so no problem here. Diff job is set to 'Append - overwrite if no appendable media'. Now I was hoping that it would append Diff backup every night till it runs out of space on '1000GB' media and then it would overwrite...but on the second day it appeneded half of the backup (35GB) to the same media (file) and then it created a new file and wrote rest of the backup to that file. First media (file) now says 'Not Appendable' and this is why it created new file...but I dont understand why it became Not Appendable...there is so much space still on this media (file) and there is no overwrite/appendable protection. I dont want a new file created...I want it to use the same media (file).

I searched for answer on the internet but couldn't find one. Can someone help please?

[RahulG]

Increase the number of backup set per bkf file by default it is set to 100 .... backup set meanse the resourse like c drive is one backup set , system state in one backup set and so on ....

[cestmoi]

That's very odd definition of Set. I thought 1 Set meant 1 backup. 
So in this case, when the file was not appendable anymore after 100 sets, it should have overwritten it because I have set the option 'Append - overwrite if no appendable media'. Why did it go on creating a new file?

[RahulG]

the makimum size of bkf file which yo have defined is 1000gb but the number backup set which should fit in that backup set is 100 , so if the you are backing up 2 backup set which is 500 it would append ,,,or if the backup data is less than 1000 gb and the backup set are more that it would  it would create another bkf file as that is what define max backup set .per bkf file ...

[cestmoi]

ok so basically we cannot use the original media (file) ever again because it already got 100 sets on it? 

I have 54 backup sets...so I guess everytime we do backup, it writes 54 sets to the media.

How can I configure Backup Exec so that it will overwrite the oldest backup when the media (files) is out of space?

[Ken_Putnam]

When writing to Disk (as opposed to Tape), I recommend that you do NOT use Append.  Make all your jobs Overwrite.  Each time you APPEND to a disk backup file, you extend the Overwrite period for ALL BKF files in that backup "family"