I also wanted to chime in with my experiences preclearing two drives over the weekend. I tried preclearing them 4 times and Unraid still insisted on clearing them when I assigned them to the array. I first tried what @Cull2ArcaHeresy did with the last preclear by skipping post read, but that didn't work for me. I rebooted, and that did not work either. After a lot of trial and error I got Unraid to accept the drives only by disconnecting the drives, going through a power up and down cycle, reconnecting the drives, and starting it back up one last time. So, if anyone else is having trouble maybe this will help someone.
And @gfjardim, thank you a lot for this plugin.