Yes it is quite possible that there is no such thing as storage storage.I really don't believe it exists at all! Even if you searched the web for hours on end I don't believe it is out there.
Yes from what i have read they are the same. The storage unit is the normally everyday storage the storage system is a more up to date service where they are more hands on.
A DVD is an optical storage device, hence the lens used to read the disc. It cannot be overridden. A video cassette, on the other hand, is a magnetic storage device. It can be rerecorded over.