Chosen Solution
I’m not looking to touch the PCIe SSD my system has. I want to know if I can just upgrade the HDD portion to an SSD instead of using the slower 7200 rpm HDD. If so, will it be possible to “fuse” the stock PCIe SSD and new SSD or will there basically be just two SSD’s seen in the OS?
In the past the answer would be no as Sierra and older OS’s leveraged the device type to set the way Apples ‘Fusion Drive’ worked. With it you needed one SSD and one HDD. Right now we just don’t know what Apple is doing as High Sierra redid the lower level structure from GUID/MacOS Extended (Journaled) to GUID/APFS. This may still be the way you’ll need to go. At this time High Sierra APFS Fusion Drives are not supported and RAID drives are also not working either. So we’ll need to wait to see what happens. Will Apple fix APFS and when? So what can I do now?? You’ll need to leave the drives independent and just make sure you set the PCIe SSD as your boot drive. Now the tricky part! The SSD Apple was using in a Fusion Drive is quite small FYI: Gizmodo - Apple Quietly Made the Fusion Drive Much Smaller So you may find it’s not able to support your OS and Apps as well as have any spare space for virtual RAM (paging in Windows). You may want to add more RAM if you only have 8 GB to off set the smaller SSD’s size. You might want to forgo setting it as the boot drive as what you are using the system may be too much for the small drive. As you where very clear you didn’t want to alter the PCIe drive you might want to look at that again or look at getting an external RAID’ed Thunderbolt SSD drive. Frankly, I would leave the system alone and go with the faster external RAID’ed SSD setup and make that my boot drive (Sierra only)
While a fusion drive using two SSD’s may have been possible with High Sierra, it is not possible with Mojave. The “coreStorage create” command only allows one HHD and one SSD to be fused. Also Mojave will not allow you to format the Logicboard SSD as anything other than APFS.
