When we mount request, it is usually due to the fact that available media is not available.

Brief explanation of how allocation works

The sequence of media allocation is in the order of the following Data Protector media sets:

1 Preallocated Media
2 Appendable Media
3 Uninitialized Media
4 Free Media
5 Overflow Media

Each of these media sets has its own definition and rules about the sequence of media.

These are explained below:
1 Preallocated Media
--------------------
Media named in the datalist device options preallocation list. Preallocated media in 'Poor' condition will not be used. The pool policy can be Strict or Loose. This media set is not sorted.Order of use: as specified in the datalist, provided that this won't break any other rules such as those relating to protection and appendable media.

2 Appendable Media
------------------
Media in 'Good' condition, with some currently protected data objects,but the media is not full. The pool must be 'appendable'.This media set is sorted according to the time of the last write.
The most recently written medium is listed first.

Order of use: when one or more media have protected objects, the most recently written media is reused first

3 Uninitialized Media
------------------
Media is assumed by Data Protector to not yet have a Data Protector header and that it can be auto-initialized as required, during backups.

The pool policy must be Loose to allow auto-initialization and the globa lfile needs InitOnLoosePolicy=1.This media set is only available in exchanger devices.

This media set is sorted with 'Blank' media ahead of media with an' Unknown' header.

Order of use:
a. 'Blank' media is used first.
b. 'Unknown' media is only used when there is no' Blank' media

4 Free Media
------------
Media in 'Good' condition with no currently protected objects. This media set is sorted according to the time of the last write.The least recently written medium is listed first.
Order of use: least recently medium is used first.

5 Overflow Media
----------------
Media in 'Fair' condition with no currently protected objects. This media will only be used if no 'Good' condition media are available.
This media set is sorted according to the total number of overwrites. The medium with the least number of overwrites is listed first.
Order of use: least recently medium is used first.

* Unclassified Media

Media in the following categories are not classified into any of the sets by Data Protector. As a result, they are not allocated for use by DataProtector.
-Media in 'Fair' condition which is protected.
-Media in 'Poor' condition.
-Media which is recognized by Data Protector as having a header for another backup utility such as 'tar' or 'fbackup'

Other Factors Relating to Media Usage
=====================================
* Strict Policy
---------------
The Strict allocation policy is not directly related to the use of a preallocation list. A pre allocation list can be used by both Loose or Strict policy. The order of media use is generally the same for Loose and Strict policy.
The difference is in Data Protector's response when the medium in the device is not the one dictated by the allocation rules.Strict policy means that Data Protector will not use any other medium than the one its own rules indicate should be used.

If the policy is Loose, any unprotected medium can be used, if found int he device.

* Protected and Unprotected Data
--------------------------------
The order that media are selected for use, depends in part on whether or not the data on the media is protected. In general, if the data is protected, a medium will be used for appending more data. If the data is not protected, it will not be used for appending, and will not be overwritten until no other medium is available.

In addition to and based on Sandro's perfect overview, please provide us with the basic information, so the situation can be explained. What is the default pool for the device that is used? What's the content of that pool? And what's the content of the library slots? You should also verify whether a pool and/or a preallocation is defined in the backup specification.