Doug S

Identical WD6400AAKS drives are not identical!

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I recently bought two WD6400AAKS drives online, intending to use them as a RAID pair. I noticed something odd when I ran a SMART self-test on both, one said it was going to take longer to run the test than the other one.

I looked more closely at them and found they have different capacities! One is 640,133,946,880 bytes and said it would take 136 minutes to complete the test, the other is 640,135,028,736 bytes and said it would take 127 minutes to complete the test.

When I run performance tests on them, the larger drive is slightly faster, getting 122 MB/sec sustained over the 16GB versus 115 MB/sec on the other. The faster one was made in Malaysia according to the label, the other in Thailand.

This isn't a real problem for me, the difference is rather small and in some ways I figure having a pair of drives that aren't identical (certainly at least not manufactured in the same lot) is a good thing in case there is a problem I'm less likely to be bit on both drives in the RAID at the same time.

But it sure is odd to me that two drives of the same model, sold together from the same online vendor (zipzoomfly.com) would have different capacities and different performance characteristics. Is this common and I was just unaware of it?

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Yes, drives of the same model can have differing performance characteristics. Of the 4 WD6400AAKS I've tested for STR, one was faster, two were about tied in the middle, and one was slower. Perhaps normal variance of motor speed (i.e. they don't spin at a perfect 7200rpm) or different brand motors were used.

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WD is notorious for this, too. It has nothing to do with bad sectors, just that WD has 2 different sizes from the factory for formatted capacity. They have a utility to change the LBA size, but I don't think it's available to end-users.

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