I noticed that I haven't been getting notifications for OS updates for the last few weeks, so I downloaded the 10.13.3 combo updater. When I try to install it, it says "macOS High Sierra 10.13.3 Update Combo can't be installed on this disk." Then it directs me to instructions on how to revert the file system on a Fusion drive to HFS+. The thing is, I don't have a Fusion drive - it's a stock, 256GB SSD. The machine is a 27" Late 2013 iMac. I've tried running it in safe mode - same thing. I reinstalled OSX from scratch using a bootable USB drive a couple weeks ago and I suspect that's the reason this is happening, but I don't know how to fix it. Any ideas?