Hi,
I use the DAR contrib in SME 7.5 to backup to a NAS box, it doesn't need to be mounted as it's a shared directory on the network domain visible to the admin user. If you want to back up to a USB drive directly attached to the SME server then you need to mount it, there are instructions on the wiki.
The target is: //NAS_BOX_NAME/directoryname
Mine is set to smbfs filesystem as it's a windows share.
My USB option is set to ext3 filesystem.
regards
John