I love how people are so quick to knock tape backups. In large scale data backup and over long term, tape backups are still a good thing. Disk/tape hybrid solutions for backup provide the best of both worlds to companies that need them.
A virtual tape library will do exactly what you are asking for. It should integrate seamlessly with your existing backup solution (if you have one) as a tape drive. Once backed up to a virtual tape (i.e. a hard drive) you can then migrate from the VTL to a real tape robot that connects to the VTL.
There are 3 major VTL solutions available. Quantum, Falconstor, and NetApp all make one. You can buy the Falconstor solution from the likes of EMC and others. I think most of the solutions start in the 2-4TB range and are easily expandable. Be prepared for sticker shock, these guys aren't cheap.
Other than that, some of the major backup software supports disk staging as an option. These just aggregate your backup data onto some disks somewhere before moving them to your tape drives. In this case, I believe you can use about any type of disks that you want, whether it be DAS, SAN, NAS, or iSCSI based.