I had a very similar issue a year ago with a previous beta of an earlier OS. The iMac has a 3TB Fusion Drive and since beta 1 is started to reboot and act strangly. I woke up this morning to find the iMac shut off and it won't boot. Ran disk untility only to find errors on the drive it can't repair. I have now wiped the drive and I am reinstalling Sierra from scratch. This is too much of a coincidence that I have this same type of issue a year ago.
Is anyone else here who has a Fusion drive experiencing odd behavior, crashing etc.. ?
I had a problem with my fusion drive, It was related to partition.
I deleted the Disk group and I use the Old Disk Utility to fix it.