If the tape has been erased clean, will incremental backup backs up everything since the tape has no existing data?
We'll assume your incremental backup is using the archive bit.
The archive bit is a marker on each file, which gets "turned on" when a file is created or modified. In Windows you can see it by viewing a file's advanced properties (It's called
File is ready for archiving).
When you perform a full backup, every file included in your selection lists (apart from those excluded by active file exclusion) is backed up, and the archive bit is "turned off" on each file.
When you perform an incremental backup, every file included in your selection lists (apart from those excluded by active file exclusion) that has the archive bit "turned on" is backed up, and the archive bit is "turned off" on each file.
So the decision about whether a file is included in an incremental backup is made by the presence of the archive bit, which is stored with the file. So in your scenario, incremental backups will not include every file.
If the tape has existing data and shows only about 1GB of memory left but is overwritable, I am still able to do backup until the tapes becomes protected, correct? Another word, I do not have to erase the tape every time I insert another tape because the current tape is full, correct?
If your tape is overwritable, then you can overwrite data on it. It won't become overwrite protected until the moment you write data to it. You don't have to erase tapes, because if the tape is overwritable, and you backup data to it, it simply overwrites the existing data.
In the last 5 years I haven't run a single erase job. Overwriting data is all you need normally.
