Local disk D: is probably on the same physical drive as C:, so it really is not recommended; it is better than nothing but it's not ideal. If that hard drive would fail/crash you could easily loose C: and D:. Also, since that drive is always accessible, any virus that deletes or edits files on C: could also erase/damage files on D: including your backups.
So if you have important data that cannot easily be replaced, for example family photos, it is always a good idea to back them up to a drive that is not connected all the time. I recommend a little flash drive you can throw in a drawer and only connect it periodically to update your data files.