When you append to a tape, it will write to the end of the tape and then overwrite the next tape.  If you don't have an overwriteable tape, when the data spill over, then you will get an alert asking you to put in an overwriteable tape.

There is no automatic facility in BE to check whether the backup data will fit on what space remains on the tape and if it does not, overwrite that tape.  You got to do this manually i.e, erase the tape before the next job starts if there is insufficient space on tape.

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As decscribed there is no way to have Backup Exec predict that it will run out of space and therefore start as an overwrite instead of an append.

However assuming that tape LTO000001 is used on a Monday and appended to by 4 backups and that you then want to use the same tape again next Monday then you can make it always overwrite - as long as you only intend to keep the previous week's data for 1 week. There are 2 ways to do this

Method 1:
a) Make sure your backup jobs for the Monday are set to Append or overwrite if there are no Appendable tapes
b) Make sure the Overwrite Protection is set to 5 days
c) Make sure The Append Period is set to 1 day

Method 2:
a) make sure the first job on the Monday is set to overwrite
b) make sure the other 3 jobs are set to append
c) Have your Overwerite protection set to 5 days
d) does not matter what your Append period is as long as it is longer than 1 day

Obviosuly if your 4 append jobs per day try to use more space than will fit on 1 tape - the only solution is to have another overwritable tape available

Note that the overwrite protection period is calculated from the end of the last job that appends to the tape.  With Coin's methods, since the last job is on Friday, the tape will only be overwriteable on the following Thursday.

I'm trying to do something similar, but I'm not backing up to tapes, but to a disk shared by another server.

I want to keep three backups, continuiously overwriting the oldest one. (Backups happen once a day)

So, to do that would this be the correct settings?

1. Set the backup jobs to Append or overwrite if there are no appendable media
2. Set the overwrite protection to 60 hours (2.5 days)
3. Set append period to 1 day

Can you tell me if this will do what I'm hoping for?

Thanks

For Disk jobs I recommend all Overwrite jobs. 

OPP of 2.5 day should work

Also from Tools/media management

if you only want three days of data, set the option to "use overwriteable media in the target media set before scratch media"

If you want at least three days, and as much more as will fit on the disk set it to "use scratch medai before  ..."  this will fill the disk up to the reserve level before overwriting old data