Actually, ARCserve's TapeEngine runs as a serive, so if your server is Win2003 or even an XP box, then a Mapped drive will not be accessible to a service. If this is ARcserve 11.0, then you can use UNC paths to create remote FS Devices, if this is an older version of ARCserve, you can runt Tapeeng.exe as an application, to get around the MS Design. Just run Tapeeng.exe from the command line as follows, make sure to stop the running service and set it to manual first:
>tapeeng -a
...the -a will allow the Tape Engine to run as an application for the logged on user and will have access to the mapped drives. Note: this will only work while the user is logged on, if you log off, the Tape Engine will stop, since it is not running as a service.