It will depend on the backup plugin. If the backup plugin supports changing the path, it will depend on how it handles it, as some plugins may lock down the ability to change the drive location, therefore you would only be able to change the location to somewhere else from the root of the server (aka where spongevanilla/forge.jar is stored).
If the plugin has not locked down the path then you should be able to specify the drive such as
And it would just save the backups to there.
As for if the drive has a different ip, such as a network drive, this (i believe) in java is programmed very differently and is not as easy to deal with, therefore the plugin needs to support this feature intensionally.
Hope this helps. If you are dead set on a backup plugin and it does not support drive changing/network drive changing, you could request a plugin that automaticly transfers the backupped files to another drive or over a network