What more information do you need to know?  Following the post I have given my RAM, my hard drives, my motherboard, and my power supply.  No one has mentioned that this could be a software issue in the kernel?  I have a 15TB system, was the software meant to handle this much?

There are a few 20 drive x 3T sized arrays out there = 60T of data storage and there are many 20 drive x 2T sized arrays out there. I think the software can handle your array size just fine.

 

I recommend you completely pull the disk that won't preclear and see if everything suddenly becomes happy. In the future, if removing the drive does prove to fix the problem, and you still decide to install it again and test it further, only do so when you have stopped the array so a crash won't mess up the array. Then test away with the array stopped.

 

Peter

 

All drives went through a preclear once, even the one that was slow to start finally went through.  I was about done with the second preclear of all drives (about 96% cleared on stage 10 of 10), when I started to get those ATA bus errors (see syslog above).  The server froze, no keyboard input, so I went to the web interface and did a powerdown.  The system did not power down, so I did a hard power down.  I have not touched it since...looking into alternatives.  I googled the "ATA bus error" and found that it was a linux kernel bug that the newest version of linux has fixed, so unRaid is running the older version of linux?  I do not think it is my hardware.  All drives got the "ATA bus error."  I already replaced two of my Sata cables, and I would look at my motherboard before I changed all six cables, including the two I already replaced. 

Post SMART reports. A single bad drive could cause this.