For many years, tape was the only way to do backups. To do them fast, fairly reliably, and to allow for offsite storage of data to cater for DR-type situations. Even with the release and improvement of USB drives, tape still had a speed advantage over the first lot of USB 1.1 drives (ever try backing up to 1 of these...?).
At this stage, both software and hardware had not matured enough in terms of low-, mid-, and high-level storage. Disk was still very expensive, and not many vendors had take the road of D2D backups for environments. Over the years this changed...the end result is that many are advocating backing up to disk first before streaming off to tape, or leaving tape completely out of the equation.
So why would disk replace tape in your environment? It's become a lot cheaper, with more advanced features like RAID offering better protection and performance, and software (as well as hardware in some cases) features like deduplication allowing for faster and more efficient backups to run. Disk can now take the form of a USB drive, a NAS, a DAS, or a SAN offering connectivity from iSCSI, to CIFS, SCSI, SAS, SATA or FC. A lot more connectivity than tape can offer. Disk-based backups are generally regarded as being faster now than tape, depending on your RAID level and how your disk has been tuned to offer more performance.
Some of the disk-types are discussed below, and are mainly what I have either tried, or are currently using in our environment:
1. DAS Storage
DAS devices can either be in the form of USB drives, or arrays connected via SCSI/SAS connections. We've used USB 2.0 drives in absolute emergencies where backups to tape have been impossible due to hardware failure. A severe limitation that I have found is the speed involved. In most cases, it has been a USB drive attached to a USB 1.1 port, and backups have absolutely crawled along. Still, a backup is better than a failure, but it does come at a cost. Backups on subsequent days have been missed as a result of backups running overtime. However, when connecting a USB 2.0 drive to a USB 2.0 port, speeds have rivalled LTO2-type speeds which are more adequate.
If you're looking for a cheap solution, USB drives are good enough (configured as Removable B2D in BE 2010 R3), but could limit you in terms of speed and failure rate...you'll only ever be able to backup to 1 drive with no protection.
DAS drives could also take the form of a RAID array connected via SAS/SCSI, or even FC if you have the money for that. Benefits here are drive speeds are faster (up to 6GB 15K SAS drives), with more protection by configuring RAID which increases redundancy. The downsides to this are more cost involved, depending on various factors like the number and type of drives required, and your vendor. For examples of these, you'd be looking at EMC's VNX 5100 (for FC block) or HP's D2000-series disk arrays (for SAS connectivity).
With DAS, however, there is no way in which to share your storage with other servers. If you had a large environment with SQL/Exchange servers that needed fast backups, you'd either have to back up to a central server (much like SCSI-attached tape) or on each server via internal disk or DAS-disk which shoots the cost of BE up with more media servers required, and adds complexity to it all in terms of management.
2. NAS Storage
This is 1 area of storage that has improved over the years. NAS devices are becoming more main-stream and seem to be bridging the gap between entry-level and mid-level storage. They're cheaper than SAN-type storage, and offer the additional benefit of redundancy in the form of RAID configurations, depending on which vendor you're looking at.
This allows multiple servers to target a NAS (via CIFS/iSCSI protocols) simultaneously, cutting down backup times substantially in some cases.
We've got 3 Iomega NAS devices in our environment (2 x Iomega StorCenter Pro ix4-200r, and 1 x Iomega StorCenter Pro px4-300r, with more on the way!). Initially backups weren't possible to it. I could never get BE 2010 R2 or R2 to access it properly, but since moving to R3, I've been able to configure backups to this device if need be. The end result has been speeds offering the same performance as the tape drive on site, and a very viable alternative to tape. I haven't tried the dedupe option offered in BE 2010 R3, as currently that isn't under investigation.
For the price, we've got good redundancy, decent backup speeds and lots of space (at least 6TB). Working out slightly cheaper than tape drives, we're in a push to get more of them in as we move along.
The downside to this is support...if your NAS isn't listed on the BE 2010 (or previous versions) Hardware Compatibility List, chances are good it won't work, and if it DOES work, any issues will get the: "It's not a supported configuration" line from Symantec (or any other vendor in the same position). Secondly, backups are still LAN-based...you're still targetting the LAN to get backups through no matter what the time they run at.
3. SAN Storage
The best of the best, if you can afford it...SAN storage is ideally best utilised with FC connectivity, as coupled with SAN SSO and servers that have very large amounts of data to be backed up, you can end up with LAN-free backups where data doesn't touch your network. SAN backups would be faster (using either 4GB or 8GB FC connectivity), allowing even more disks to be targetted, with the ultimat configuration being able to stream off to either another array or an FC-connected library which completes the LAN-free backup process.
Cost here though is a major consideration. SAN attached arrays from companies like IBM, HP and EMC can cost a lot more than other storage, so the performance/cost issue needs to be properly considered.
We used SAN SSO to backup a file server with a couple million files and around 600GB of data in total, along with an Exchange server with 4 x 100GB Information Stores and it cut backup times down by around 60%. Times will vary, so don't quote me on that applying to your environment, but it saved a lot of time in the long-run.
So where does tape fit into all of this? Off-site storage for backups can be taken care of by doing a D2D2D strategy where data recently backed up to disk is duplicated to another array elsewhere on your site, or across the WAN. No tape needed here.
Cost vs. performance...tape in my mind is a better option than disk if you're considering a USB drive for instance, or for some reason have a NAS not on an HCL which rules it out of the equation.
Some auditing firms and other companies require offsite storage where data is stored in a safe, either in another building or at a storage company. Disk wouldn't do the trick here, and tape is still viable. You could still look at following a D2D2T policy where backups are first done to disk, and then streamed off to tape before it is vaulted.
Tape isn't entirely dead, and the way the LTO format is being constantly revised, it might never be. But it might very well be playing less of a primary role in backups and eventually be relegated to vault/second-tier, or even third-tier storage, especially as disk becomes cheaper in all formats.
However, if you're not considering disk in your environment, you should be...it's the way of the future (in my opinion).
Why not check out Backup Exec 2010 R3 if you haven't already and see what it has to offer...