Boot camp stuck on Partitioning disk...

Trying to install Win10 on my iMac 27 late 2015 with 1TB Fusion drive running Mojave 10.14.5

but every time it get stuck in "Partitioning disk..." with the progress meter at about 50%.

I've tried both October and April versions of Windows with the same result. And I've tested creating a FAT partition in the disk utility and that worked. Also tried erasing the entire drive and re-installing Mojave.