> Now, suppose a file is loaded to the SSD and we never write to it. Now the SSD gets full, and the file has to be moved off of the SSD to make room. On a typic

> Now, suppose a file is loaded to the SSD and we never write to it. Now the SSD gets full, and the file has to be moved off of the SSD to make room. On a typical HHD, all that happens is the file gets deleted from the SSD. It's still on the HDD, so no further action is necessary. But on Apple's Fusion Drive, this file will have to be copied back to the HDD. I guess that in this marginal case, the Fusion Drive does behave rather like a cache and allow the blocks simply to be overwritten if the data on the HDD was not overwritten - as you say, it would be inefficient not to, if simpler. But if the Fusion Drive allows the data on the HDD to be overwritten before that, then the FS has spare capacity that a proper cache cannot use, since a cache cannot free up the space on the HDD. The result is that the space available for data is close to the sum of the space available on both drives, something not true of cache.