RMAN fails on mapped drive

Hi,on 8.1.7  on Win 2003 my rman script is :

 BACKUP

    INCREMENTAL LEVEL=0

    FORMAT '\\REMOTESERVER\BACKUP\RMAN\FULL\MYDB\bk_u%u_s%s_p%p_t%t'

    FILESPERSET 6

    SETSIZE 29000000

    DATABASE;

And I receive the following error :
Here is error stack :

RMAN-00571: ===========================================================

RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============

RMAN-00571: ===========================================================

RMAN-03007: retryable error occurred during execution of command: backup

RMAN-07004: unhandled exception during command execution on channel ch00

RMAN-10035: exception raised in RPC: ORA-19504: failed to create file "\\REMOTESERVER\BACKUP\RMAN\FULL\MYDB\bk_uh6os0eg9_s270886_p1_t834681353"

ORA-27040: skgfrcre: create error, unable to create file

OSD-04001: invalid logical block size (OS 512)