SSD slows when changing Bay (between main bay and optical bay)
SSD running OS X [Sierra or High Sierra] can be made to perform well [= read/writes at 400-500 MB/s ] in both Sata III bays (whether the main bay or the optical drive bay).
Problem: When moving SSD from one bay (main bay or optical drive bay) to the other, it would perform very slowly. The SSD seems to work fine in whichever bay the SSD was in when it was originally installed (whether the main bay or the optical drive bay). It was only when the SSD was moved to the other bay that I began to see highly variable read/write times, which at times were normal [400-500 MB/s ], but most times were very [<100 MB/sec]. Running Sierra and High Sierra (problem manifests in both versions of OS X).
Note 1: I have tested both my main bay and my optical drive bay, and both are Sata III (I have achieved read/write speeds between 400-500 MB/s on both bays).
Note 2: The problem is not SSD specific. For example, I have built OS X Sierra on a Samsung 850 (500 Gb) in the optical drive which functioned fine [400-500 MB/s ] until it was moved to the main bay, it became too slow to be useful. When I reinstalled OS X Sierra on the same drive using the Sata connector in the Optical drive if functioned fine [400-500 MB/s ], until I moved it to the main bay, at which point it became extremely slow. I have repeated the same experiment with a Crucial MX 500 to the same effect.
Note 3: Seem to have the same problem whether in Sierra or High Sierra
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