Yeah, DASH copy would be the cleaner way if you can afford the extra space that will be used to create it. In my case, it was in a remote office and I didn't have any room left to create another copy.
For the offline drive, if you've updated the mount path manually and it's still showing offline, try restarting the media & library manager service on your Commserve, this should force it to update its status and see the new path so that the path can be shown as being online. Doing a DASH copy won't work until the path is online anyway. If after restarting the service, it still shows as offline, make sure to double check the path you've manually entered in Simpana.
Also, if you have 2 paths, it's a bit weird that your backups wouldn't run if one of them is offline as they shouldn't require both paths to be online to run their backups. While one is offline, it should be able to write everything to the other mount path granted it has enough space/writers.