Having seen quite so many errors writing to the disk on Slice I decided to dig in a little deeper and see what's going on. It turns out that the filesystem was only getting flushed back to the disk when it was unmounted. This means that the only time that the disk was not going to lose data is when you completely shut down properly and pull the power.
I've been in communication with the writer of the exfat-fuse driver that we use and he suggested a patch to improve flushing of data to the disk. Basically what this means is that for a small amount of performance loss (only for disk writing not for reading) we have a massively improved reliability to accidentally powering off the slice (power loss, random bugs or just accidentally turning off the wrong switch!)
I've checked this into Nightly but think it should go very soon to Stable because it should be a significant improvement in reliability!