Use sp_addumpdevice to add a backup device:
sp_addumpdevice{ "tape" | "disk"} , logicalname, physicalname, tapesize
where:
physicalname can be either an absolute path name or a relative path name. During dumps and loads, the Backup Server resolves relative path names by looking in Adaptive Server’s current working directory.
tapesize is the capacity of the tape in megabytes. Most platforms require this parameter for tape devices but ignore it for disk devices. The Backup Server uses the tapesize parameter if the dump command does not specify a tape capacity.
tapesize must be at least 1MB and should be slightly below the capacity rated for the device.