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").