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I'm considering to replace my optical drive in my MacBook Pro with an SSD. When using it as separate Volume, the advantage is that you can spin down the HDD, thus reduce noise and first and foremost, battery consumption. Manually managing the files however is a nightmare IMO.

Setting up a Fusion Drive solves this problem, however it seems that the HDD is constantly spinning whenever you access the Fusion Drive, regardless wether the data sits actually on the SSD portion or the HDD. See here (the mentioned ArsTechnica test was done on a Mac mini, question is whether the Fusion Drive managing behaves any different on portable Macs with different Power Management settings).

Question: Is there any way to force the FusionDrive manager (coreStorage?) to spin down the HDD, when the data comes off the SSD and the HDD has nothing to do?

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I just learned that the Fusion Drive "technology" is a lame duck, not a serious caching solution taking full advantage of the SSDs capacity. According to this credible report "the algorithm seems to be: Keep 4GB SSD space free, push down items to the hard drive to maintain that 4GB.", which means once the SSD is filled, there is no optimization in terms of any frequently used data -> SSD, any untouched data -> HDD going on. This renders my above question, at least to me, obsolete. –  Dr. Woo Feb 5 '13 at 1:33
    
Because of this Fusion Drive is not the path to choose. Unless there is some true software caching available, splitting the contents with System, Apps and Caches sitting on the SSD, and User data residing on the HDD looks like the much more promising approach. See this video for instructions on how to set this up. –  Dr. Woo Feb 5 '13 at 1:41
    
I wouldn't call MPG "credible". See this article or this post that both describe observed HDD->SDD promotion, including details on methodology (something the MPG article lacks, as usual). –  Václav Slavík Feb 13 '13 at 10:44
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