I'm still a fan of tape. I know it's "dying" but for capacity, longevity, and portability, it's pretty tough to beat. However, I have some clients and prospects that have a "hate-on" for tape and want to move to disk-to-disk backups. I've typically setup disk-to-disk-to-tape to give you that quick recovery time for "every day" restorations (like corrupt or deleted data), but the portability of tape that you can take with you off-site (and just as importantly, off-line) in case of disaster.
I've setup an e-SATA enclosure with a two-disk rotation for very small clients that couldn't afford a tape drive; I'm debating using this for a SAN-backed virtualization setup that still requires as per policy, an offline/off-site backup (even though they have a DRP "warm" location that's replicated via Veeam Backup software to an ESXi host across town), in particular the "off-line" bit to mitigate against tampering/malicious activity, etc.
I know Dell makes a disk-based backup drive that uses disk "tapes", but I'd prefer something that can be recognized as a storage device in Windows, Linux, etc. and mounted accordingly.
In my head, this unit would be small, lightweight (perhaps using 2.5 in drives), rugged (with a durable tray/sled for each drive), work with standard SATAII/III drives (or possibly SAS), with a corresponding external interface. Nice to have would be some redundancy (via RAID1 "pairs").
Take a look at TapeSucks.com. I am using the 2 Bay Tandem AMT 1U rack mount system with 2TB drives. I take home one of the drives every night for backup. This does not solve keeping backups for archiving purposes, however this does solve our need for disaster recovery and deleted files for the last year.
If I'm reading what you're looking for right, you'd want something like this from Buffalo. It's a 4-drive external unit that has USB and e-SATA capability with RAID functionality. We use this in house at two of my sites, and I haven't had any problems with them. Standard YMMV disclaimer applies.
This looks promising: no redundancy and 2.5" drives would be welcome, but this is certainly a step in the right direction, at least from a portability/physical durability perspective. And with a regular disk rotation/replacement policy (based on number of write-hours and/or elapsed time), I could see this functioning quite well:
http://www.lacie.com/ca/products/product.htm?id=10382