I got my iMac back from the shop (we don't have Apple shops here in Thailand, but it is an authorized service center) with a new WD Black 1 TB drive installed, and I used the Migration Assistant to restore the computer to its previous state. Everything restored just fine, however, the computer now seems quite sluggish. It seems to take a long time to boot and to load apps. I'm wondering if this has to do with the fusion drive not being properly configured or perhaps in conflict with the cache on the WD drive. Here is a capture from the Disk Utility. Is this OK or do I need to do some configuring?