Describe the bug
I just recently wanted to buld a server room with a lot of drives. I layed out smart and dense smart cables to connect them all but after I placed a few of the drives, I noticed that some were not connected, even tho a cable with enough free channels was attached to the back.
I also noticed that the channels went out from only one drive. So it seems like, the lastdrives are not connected because AE wants to conecct them only via the inter-drive-connection and not through the cables in the back.

To Reproduce
Just place drives in a 3x3 grid, attach smart cables in the back like shown in the picture

Expected behavior
all drives sould be connected through the cable in the back of the drive

Environment
Playing Project Ozone 3 Kappa Mode

• Minecraft Version: 1.12.2
• AE2 Version: rv6-stable-7
• Forge Version: 14.23.5.2847

okay, thanks!
I'll try that out as soon as I fixed my world from crashing

This is just how it works, searching for the shortest (optimal) path.

but it's not optimal when there are no free channels left in the path

I would say try to reconnect your connections. I tried it and it works with many possibilities (yours included), and they work just fine.

Few images for you:

At the end the connections sometimes work little wonky and they do not prefer cables over machine-to-machine connection