by eugene

What’s the difference between the WD4000YR and WD4000KD?

Also known less formally as the Caviar RE2 and Caviar SE16 respectively, this pair of 400-gigabyte offerings from Western Digital has generated quite a few questions across the SR Discussion Community. Both drives share a similar set of features yet one seems to be oriented for enterprise use whereas the other seems to target the desktop market.

Our review of the Caviar RE2 uncovered the drive's stellar performance and has readers asking if they can expect the same from the 400 GB SE16. So, what exactly are the differences between the WD4000YR and WD4000KD? There are only three:

1. First, the WD4000YR (that is, the “Raid Edition 2”) enjoys a 24-hour factory burn-in period versus the WD4000KD (the “Special Edition 16”)’s 8-hour test. The longer validation cycle increases the chances that a drive destined to suffer an “infant mortality” of sorts is caught at the factory before it enters distribution.

2. Next, the YR features a longer 5-year warranty more typical of enterprise-oriented drives such as WD’s own Raptor or Seagate’s Cheetah series rather than the shorter 3-year coverage that backs the KD.

3. Finally, the RE2, living up to its moniker, ships with the firm’s “Time Limited Error Recovery” (TLER) feature enabled while the SE16 does not. This oft-confusing feature permits a drive to gracefully surrender error recovery features after an 8-second window of attempts rather than stubbornly continuing onwards with the assumption that an array’s redundancy features will cover the error and avoid data corruption. For drives that find themselves in a setup that features redundancy (anything other than RAID 0), this is a plus. For disks that don’t, it’s not.

So, in the end, both these devices are physically and electronically the same piece of hardware. They come off of the same assembly line and are subsequently treated separately via the differing burn-in times and factory TLER settings. This is borne out by SR's own perfomance testing of the SE16.

Though spec sheets fail to mention it, the 400 GB SE16 supports NCQ. It’s the same physical drive as the RE2, a drive targeting the multi-user market where such a feature is a necessity.

The RE2 (and thus the SE16) shares a significant amount of electronics and parts in common with the 10K RPM Raptor family. Thus, the 400 GB SE16 comes equipped with a more conservative signaling scheme that limits its interface to 150 MB/sec rather than the 300 MB/sec found on the 250 GB SE16 (the Caviar WD2500KS, a fundamentally different design based on the older Caviar electronics which also does not support NCQ). Remember, of course, that these are maximum rates- sustained rates from the drives themselves don’t come close and really aren’t limited by either of these figures.

This leveraged technology between the RE2/SE16 and the Raptor leads WD to boldly proclaim that the WD4000YR can sustain use in a 24 hour a day, 7 day a week environment with its actuator seeking 100% of the time… the only such claim for any 7200 RPM drive on the market. Competing drives such the Seagate NL35 and Maxtor MaXLine Pro, while offering 24x7 power up, are more cautious in duty cycle (that is, seek) claims- “nearline duty loads” for the Seagate and “40%-50%” for the Maxtor.

WD claims a 1.2 million hour MTBF for the RE2 and a 600,000 hour MTBF for the SE16. Why the difference? Can weeding out drives that may prematurely fail with a burn-in period that runs 24 hours rather than 8 really account for such a difference? Probably not- much of this gap may be attributed to marketing. An SE16 is an RE2 that received less burn in time.

Does this mean that the 400 GB Caviar SE16 should be able to handle the same strenuous duty cycle that WD claims for the RE2? The manufacturer would probably cringe at the thought. What do you think? :)