Wise words from http://www.arcanadev.com/support/kb/I02000003.aspx:

Programs run by the Scheduler generally will not have access to network drive letters unless those letters are mapped by the Scheduler (see next section).

This is because drive letter mappings (e.g., those assigned using Windows Explorer) are defined at the user level, not the system level. These drive mappings are only valid while the user who made them is logged into the computer interactively.

Even if you are running scheduled programs under an account for which drive mappings are defined, the programs will not have access to those mappings, because the Scheduler does not load them when it logs on to that user account.

For Windows Network resources, if you are running the job under a user account, and that user account is also logged into the computer interactively, the drive letters for that user account may be available to scheduled programs. However, you should not rely on this being the case, because your scheduled programs will then fail if the user is not logged in interactively, or if another user is logged in.

Instead you should always use UNC designations for network resources, or have the Scheduler map the drive letters.

There is more well-written, useful info at that site..