Tandberg Data has given Symantec Backup Exec the push from its RDX removable disk drive line and is buying in Yosemite's Filekeeper CDP (continuous data protection) software instead. RDX has also been updated by its developer, ProStor, to include 500GB cartridges.

The result is a set-up that can not only be used for system backup, but you can also boot from it to do bare-metal restores, and it works as a file archive too, so you can go in and retrieve previous versions of a file. 

Neither technology is strictly new, but the combination of the two shows the sea-change underway in data protection - and is potentially another nail in the coffin of low-end tape. 

Quite simply, disks - every RDX cartridge contains a complete 2.5 inch hard drive, in a shock-protected mounting - have different properties from tape, and people use them in different ways, says Yosemite boss George Symons. 

For a start, we don't usually leave a tape permanently in the drive. Instead, a tape (probably from a rotating backup set) is loaded, the backup job is run, and then the tape is taken out and put somewhere safe.

Meanwhile, disk backup devices are normally connected all the time. Incremental backups can be taken much more frequently, even continually - that's a large part of what CDP is - and restoring a single file from an old backup to disk is trivial compared to doing the same task from tape.

That doesn't mean disks can't also be used for full backups though, Symons adds - especially if they're removable.  

"We take advantage of the ability of RDX to be both removable and spinning, to do both CDP and full system recovery," he explains. "It means you can go back through versions of files. We use deltas [storing only the differences between versions] and a file-level intercept driver, it's a simple idea but it's not easy to do."

He says that, with RDX, people usually have multiple removable disks for different purposes - perhaps to use on different machines, or for different purposes, or simply because their data spans two cartridges. 

MacClain Buggle, Tandberg Data's product manager, agrees - he says that while the ratio of RDX docking units to drive cartridges used to be 2:1, it's now 3.5 to 1, with 90,000 docks and 250,000 cartridges sold so far.

"We've researched the use - people typically have one in the device doing CDP, one nearby for scheduled system backup, and one offsite," he says, adding that this is why the company decided it needed to bundle a different piece of software.

"We initially went to market with Backup Exec," he continues. "That's fine in the server market, but it's only satisfying a small piece of the available market. We needed Filekeeper with RDX to have cartridge management and disaster recovery. It's also easier to set up and use when you don't have many IT staff."

He points out that while there's 800,000 low-end tape drives out there, and that's a flat market, there's 200 million external hard disks and the number is growing. So using CDP software to reposition RDX as more than just a tape replacement could enable it to break into that much bigger opportunity. 

"This doesn't replace the external hard disks used for extra storage space and so on, but it could replace the ones used for backup," he says. 

He cites IDC figures which say that RDX, which was developed by ProStor and licenced to Tandberg Data and Imation for manufacture and marketing, has some 90 percent of the market. Its main competitors are EMC-Iomega's Rev, which is more like the removable hard disks of old, in that the cartridge contains only the disk platters and motor, and the Quantum GoVault, which - like RDX - has a complete 2.5 inch hard drive inside each cartridge. 

(There's also the iVDR consortium, which is aimed more at audiovisual applications and hasn't shipped much product yet.)

RDX cartridge sizes will continue to grow, Buggle says - 300GB ones were seriously expensive last year, at $3 per GB, but are now a third of that price, and with 750GB models due next year, the current 500GB top-of-the-line will follow suit. Solid-state disk (SSD) versions could eventually appear too, although still in the same cartridge format for interoperability. 

"$300 a cartridge sounds a lot, but three cartridges and a dock is only $1000 or so," he adds. That's less than you'd pay for an LTO-3 tape drive on its own, without any tape cartridges.

However, he says that there will still be some users who'd be better off with tape, and suggests that the break-point is probably around six to eight cartridges - once you have more than that, you're probably doing enough archiving to justify a high-speed streaming tape drive.