Are these new drives? Have you tried to reseat the cables to the backplane? ftp://ftp.dell.com/Manuals/all-products/esuprt_ser_stor_net/esuprt_cloud_products/poweredge-c6100_Owner%27s%20Manual_en-us.pdf

Are the BIOS and firmware up to date on each of the systems? http://www.dell.com/support/home/us/en/04/product-support/product/poweredge-c6100/drivers

I have tried reseating. This has occurred on multiple different nodes, but so far no more than 1 slot is affected on any given machine.