This article is from the storage FAQ part2, by Rodney D. Van Meter with numerous contributions by others.
DEC's old Tops-20 OS supported offline files, and would generate an automatic request to the operator to mount a tape when the user accessed the file. When you listed a directory, it would show you which files were online and which off. DEC's OpenVMS has some sort of support for this now. VMS 6.1 supports "shelved" files. There is also the product Virtual Branches, from Acorn Software, which does HSM for MO and CD-ROM for OpenVMS. Acorn Software, Inc. 267 Cox St. Hudson, MA 01749 voice: (508)568-1618 fax: (508)562-1133 Internet: email@example.com