Imagine an external DVD +/- R/RW drive mechanism with two storage spindles and a mechanism to move disks from spindle to spindle that handles the following use case:
"The user wants to back up some massively huge drive or collection of files onto one or more cheap blank DVDs, spanning multiple disks
if necessary, without having to personally be there to swap disks and/or format/erase them when the current disk is full"
The drive's behavior depends upon whether or not the application explicitly indicates to the drive that it understands its special capabilities.
Normally (in "dumb" mode, where the driver doesn't think the app really understands how it works), the user would load a stack of blanks onto the source spindle, remove all the disks from the destination spindle, and hit the drive's "start" button, which would grab a disk from the top of the source stack, load it into the read/write mechanism, and notify its driver that a disk was just inserted. At that point, the driver would proceed to UDF-format (if blank) or erase (if formatted but nonblank) the disk, then notify any interested observer (Windows Explorer, the backup app, etc) that a formatted & empty disk was just inserted. Whenever the driver is told to eject the disk, it removes the disk from the read/write mechanism and puts it onto the destination stack, loads a new disk into the read/write mechanism from the source stack, formats/erases the disk, and notifies any observers that a formatted & empty disk was just inserted.
In "dumb" mode, the disks move in only one direction... from source to read/write to destination. The burden is on the user to avoid doing anything stupid, like putting disks that shouldn't be erased on the source spindle.
At the ghetto end, you could use Winzip or Windows Backup in multi-disk spanning mode. At the middle end, a slightly more sophisticated app might use NTFS compression and recursively back up one directory at a time (making some intelligent optimization trade-offs to strike a balance between laying down the directories in a logical manner that would make sense to a casual recovery effort that entailed browsing the disks one at a time without leaving a disk 90% blank just to accommodate one singularly huge file). This wouldn't be efficient or a good choice for corprate backups spanning hundreds of disks, but the convenience of having direct access backed-up files directly from individual disks from ANY DVD-rom drive might outweigh the added media cost for home users who'd be looking at a whopping $5-10 for the extra blank disks.
At the high end (say, in a small business), a proper backup program that understands how offline storage works could work in reverse: put the stack of disks (ideally, in order... but the drive could plough through them one by one to identify them if necessary) in the destination spindle, and let it automatically move disks from source to destination spindle to get the one needed at that moment to restore a file. Since a savvy program would interact directly with Windows offline storage mechanism, wantonly mixing blank with good disks in any random order among the two spindles would be sub-optimal, but nonfatal as long as there were sufficient room to stack ALL of the disks onto a single spindle (if necessary, the driver would plough through all the disks one at a time to identify them and subsequently keep track of their position in the two stacks).
The main beneficiaries would be high-end home users whose backup needs grossly exceed the storage capacity of even a half-dozen blank DVDs, but can't afford to spend thousands of dollars on a high-end tape drive with changer. Once a year, users could load up the drive with 20-50 DVD+R disks to do a full backup, then let the drive do differential backups to DVD+RW disks every night (or every day while he's at work). When the number of differential disks becomes unwieldy (or a trip out of town every 3-4 months presents a golden opportunity to let the drive work on the backup for a day or two), another full backup is done (on RW disks, most likely).
Hardware-wise, this doesn't seem like it would be such a big deal. Spindle-fed CD-ROM jukeboxes already exist... so it's just a matter of replacing the CD mechanism with a DVD writer mechanism, and updating the firmware and drivers to handle the new capabilities. The main firmware innovation would be assuming that a request by some app to eject a disk really means, "fetch a new blank disk, format/erase it automatically, then act like a phantom user inserted that disk into a normal DVD burner and closed the drawer".