I've always seen SCSI drives sized for an uneven capacity like 36.7, 73.4, or 146.8 GB, but SATA drives always have even capacities like 80, 120, or 250 GB. Why the disparity? Is there some technical aspect of the SCSI interface that makes those capacities easier to implement?

And why did they finally break the trend with 300 GB drives? I would have expected the next size up to be 293.6 GB.

Edit: clarifying that question is about SCSI vs. SATA

  • 300GB isn't a close approximation of 146.8GB x 2? try rounding the previous size up... – quack quixote Nov 9 '09 at 6:28
  • quack: Yes, it is a close approximation. But in the past, it was always exactly double. I've clarified my question accordingly. – Nic Nov 9 '09 at 6:58

Probably, the difference you're seeing is that most ATA drives are consumer-level, and so are marketed in consumer-level terms, whereas SCSI drives are pro-level, so are marketed in full technical detail, so that people know exactly what they're buying. That can matter for RAID setups, for instance.

It's also possible (but less likely to be relevant here) that SCSI drives are keeping more space aside for emergencies. Most modern drives keep some reserved blocks, in case the visible blocks get damaged. The damaged blocks are remapped to the reserved space, and the drive goes on working as if nothing happened. Until you run out of spare blocks :)

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    Or, to be less gentle about it, ATA drives lie about their capacity -- and so do the 300GB SCSI/SAS drives. – womble Nov 9 '09 at 7:12
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    yes, SCSI is measured in "actual" size, not "convenient" size – warren Nov 9 '09 at 9:53
  • To clarify, SATA and IDE drives are usually marketed with 1GB=1000MB; SCSI is marked as 1GB=1024MB. The reason they changed is that non-technical people (who aren't generally familiar with base-2 arithmetic) were complaining because they couldn't make the math add up. – Ed Leighton-Dick Nov 25 '09 at 16:29

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