Backup Exex 2010 R3, 13.0 rev.5204(64bit), SP2, Hotfix 180429
Backup job setting: Append to media, overwrite if no appendable media is available
Media Sets setting: Infinite-allow Append
The media data is used 30% only, but the media was overwrite
No any setting change before, but the overwrite problem occurred this time, message as below:
"No appendable media could be mounted. Switching to overwrite operation on scratch media."
The 2010 version is pretty old but the way we write ( append and overwrite) to tape media has remained the same. Please check this technote veritas.com/support/en_US/article.100017917 and see if it helps.
By any chance does the tape show end marker unreadable when the job switches to overwritable media?
If your job needs more then one media, then we need an overwriteable tape: see the following paragraph from the TN Gurvinder shared: