I have 6 sata drives which I use for different backups/storage etc.
They are all internal Western Digital drives which I connect to my laptop using USB connectors.
I have 4 USB 3 ports on the Laptop.

Four of the drives are 2.4" (320GB-1TB) and I connect using SATA/USB convertors which are self powered from the USB port on the laptop. I can connect all four at the same time with no problem.

I can also have 2 of the larger 3.5" drives (both 2TB) which have to be connected using powered USB caddies (there is insufficient power in the ports to run these.)

I can connect 3 of the 2.5" and 1 of the 3.5" at the same time with no problem.

The weird issue is that if I connect the 2 3.5" at the same time (either on their own or with up to 2 of the 2.5" drives) win 8 sees them, connects to them, allows me to browse them - for about 30 secs, then it drops one or both, then reconnects them, and then drops them again. This continues until I remove one of the 3.5" and then everything is back to normal - until I try reconnecting the second 3.5" drive.

I have never experienced this behaviour before - anyone any ideas why this happens???