Here is what happens: I plug 6 cards in and everything runs perfectly. I boot up with 7 cards and start claymore. As soon as it tries to download the DAG files, It spits out an error message for a random card. Sometimes the RX580 in slot one has a problem, sometimes it is the RX470 in slot 6.
Have you only one PSU? If so than your rig has not enough power for whole 7 GPUs. It's a miracle that system works well with 6GPUs with this PSU
Atleast it takes 1250 PSU due to power efficiency and supply for the graphics board. I haven't tried the mining yet but this information I got it from room mate who is doing GPU mining on ethereum coins. Each rig contains 6 graphics card with the one raiser connectivity to increase it big numbers.
So I replaced the PSU with a much more powerful one, and nothing has changed