I know this is an old thread. I've been struggling with what I'm about 99% sure is a failed SSD drive in the fusion drive setup (did lots of debugging using diskutil both cmd line and gui, recovery, etc). Have spent lots of time trying to get around it. I use Window about 99% of the time also. With the SSD failed, I saved the Bootcamp partition but nothing from the Mac OS partition. Plan was to somehow use that img again. Preference is to not have to open my iMac but it's looking more like I'm going to have to (if I want bootcamp). I'm at the point where I can't even load a new bootcamp on a fresh Mojave installation. The mojave installation didn't seem to care about the failed SSD. It just installed to the HDD. Bootcamp/Windows goes through most of installation but then won't boot after it installs. It bluescreens with an AppleSSD.sys failure. Go figure, it somehow is trying to use the failed SSD. Ugh. Will NEVER go the route of a fusion drive again. I'm still not able to get bootcamp working even from all fresh installs at this point. Supposedly you can install bootcamp on an external drive, so I might try that next but I'm fully expecting it to STILL complain about the SSD for whatever reason.
In case anyone is interested, this late 2015 27in iMac uses a 24GB SSD for the fusion drive setup (1TB model). If you want REALLY big speed increases, you can not only switch out the internal SATA drive for an SSD, you can also switch out the PCIe SSD drive, and that drive can run much faster than the SATA 3 since it uses the PCIe interface. I might actually do that, but, again, am not looking forward to the difficulty in taking the iMac apart (I've watched some youtube videos).
update: I was able to install and boot the Mac OS from an external drive. But the real solution came when I simply renamed the applessd.sys driver that Windows (in bootcamp) was complaining about (and the backup of that file also). It wasn't exactly simple, I guess, since it's an NTFS partition. Had to go through a bit to mount it read/write on Mac. But I save my bootcamp partition and was booting again on windows with a fresh Mojave installation. Too bad I couldn't save the Mac OS partition though. Lost quite a bit there.
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I know this is an old thread. I've been struggling with what I'm about 99% sure is a failed SSD drive in the fusion drive setup (did lots of debugging using diskutil both cmd line and gui, recovery, etc). Have spent lots of time trying to get around it. I use Window about 99% of the time also. With the SSD failed, I saved the Bootcamp partition but nothing from the Mac OS partition. Plan was to somehow use that img again. Preference is to not have to open my iMac but it's looking more like I'm going to have to (if I want bootcamp). I'm at the point where I can't even load a new bootcamp on a fresh Mojave installation. The mojave installation didn't seem to care about the failed SSD. It just installed to the HDD. Bootcamp/Windows goes through most of installation but then won't boot after it installs. It bluescreens with an AppleSSD.sys failure. Go figure, it somehow is trying to use the failed SSD. Ugh. Will NEVER go the route of a fusion drive again. I'm still not able to get bootcamp working even from all fresh installs at this point. Supposedly you can install bootcamp on an external drive, so I might try that next but I'm fully expecting it to STILL complain about the SSD for whatever reason.
In case anyone is interested, this late 2015 27in iMac uses a 24GB SSD for the fusion drive setup (1TB model). If you want REALLY big speed increases, you can not only switch out the internal SATA drive for an SSD, you can also switch out the PCIe SSD drive, and that drive can run much faster than the SATA 3 since it uses the PCIe interface. I might actually do that, but, again, am not looking forward to the difficulty in taking the iMac apart (I've watched some youtube videos).
update: I was able to install and boot the Mac OS from an external drive. But the real solution came when I simply renamed the applessd.sys driver that Windows (in bootcamp) was complaining about (and the backup of that file also). It wasn't exactly simple, I guess, since it's an NTFS partition. Had to go through a bit to mount it read/write on Mac. But I save my bootcamp partition and was booting again on windows with a fresh Mojave installation. Too bad I couldn't save the Mac OS partition though. Lost quite a bit there.