I am new to using tape drives in linux, and have a couple potentially very dumb questions.

I am using tar to back up various directories to tape. I understand how to do this, and how to move the tape to next file etc...

My first question is how to overwrite data on the tape?

I will use an example for this.

I tar a 100 MB folder onto a tape (using /dev/nst0).
I then tar a 50 MB folder to the same tape (using /dev/st0).
Then next day/week/whenever, I want to re-backup those folders, BUT, the 100MB folder is now 60MB.
Do I need to fully erase the tape before I do this? Or can I just tar the folder over the current one? If yes, how does the tape know where to fast forward to when I ask it to fast forward to the 2nd file? How does this work?

Please assume that the tape is not large enough for me to simply add the directory after the first 2 tar archives.

A second question, after archiving the 2nd folder (the 50MB one) using /dev/st0, does it rewind to the beginning of the tape or the beginning of that file?

Before anyone tells me to use google, I would like to say that I have. I have also looked in several linux books and have not found my answers. So I also ask for any links or book recommendations that have good info/tutorials about tape drives in linux.

For other people who may later view this post, here are the best resources I have found:
Oreilly Essential System Administration - 3rd Edition