We have a server with 3 TB of data. We have a holding disk of size 500 GB. We are using HP LTO-4 tapes with tape size of 800 GB. The tape cycle is 20 tapes. We are experiencing regular tape failure message from amanda.

*** A TAPE ERROR OCCURRED: [No more writable valid tape found].
Some dumps may have been left in the holding disk.
Run amflush again to flush them to tape.

I have two specific queries related to this issue
a) How can we have tape failure when size of holding disc is way less than the tape size ?
b) Does amanda append new dumps to the existing data on tapes which were used in previous cycle ? Or it first cleans the tape and then pushes the dumps ?