Hi,
If this drive is a 'free-standing' unit, such as a MyBook etc., then it cannot be seen directly by WHS. For it to be backed up, it would have to be connected to a Client computer, and have been asigned a drive letter.
In that situation, it would be backed up as part of the Clients' nightly backups, and after the initial one, further backups would only consist of changes. However, it also wouldn't be seen directly by anyone not on that Client, unless you map that drive on other computers.
If you transfer the contents to the Shared Folders on the server, you can have those folders Duplicated if you have more than one hard drive in your server. In this scenario, you can select the read/write options for each Client computer. (also, if this drive you have, is say a USB drive, then it could be incorporated into the system as a backup drive for your Shared Folders which could then be rotated off-site).
If you go down the Shared Folders route, then they are just that, folders. They aren't then backed up as well , but are just accessed as you would any other folder, so things like 'incremental' 'differential' don't apply.
Ther is quite a lot of documentation for WHS, available
here, or
here.
HTH,
Colin
If anyone answers your query successfully, please mark it as 'Helpful', to guide other users.