I'm having the exact same issue as Johnny G talks about here. Except my log files are slightly different - see attached.
I have tried on two (2) WD Red 4TB drives and one (1) WD Black 1TB drive with the same result. A window quickly flashes up and then it sits at "Starting..." indefinitely.
Server: Supermicro X8DTL-iF, dual Xenon X5670
I was last able to successfully run a preclear on Aug 24 (according to my last preclear_report file) and it finished without error.
preclear_disk_log.txt
diskinfo_status.txt
diskinfo_json.txt
diskinfo_log.txt
Update (Oct 18 2017): I found that by reverting to unRAID 6.3.0 that I could successfully run a preclear.