How does an Overwrite affect previous tape data??

[McNassar]

I was wondering if anyone could help me out in understanding how an Overwrite operation affects existing data on a tape.  For example, I am running daily backups onto a tape and want to start overwriting once the tape is full.  In testing this, I've noticed that my Used Capacity on the tape is the size of one backup worth of data (after the overwrite).  Does this mean that I cannot access the previous data on the tape?  In other words, If I have 8 days of backups on the tape, once an overwrite occurs on day 9, can I access days 7 & 8 or only day 9 now??

[Ken_Putnam]

Tape is sequential access, not random access, like disk

That means that when you start writing at the beginning of a tape, anything that may be on the tape is overwritten, and when the job ends, a new End-Of-Data marker is written to the tape

The old data is still there, unless you have done a Full Erase, but it would take a Data Recovery service to get it back.  As far as backup exec is concerned, only the data written since the overwrite is there

[McNassar]

Thank you