Its faster to use each HDD on its own
Also, it would be nice to get some performance tips for mining (NOT generating plots, there are lot's of tips regarding generating plots already present in this thread) ... are there any factors, that affect mining performance, besides the total storage space in the plots?
Does it matter if pocminer runs on a slow or fast computer?
Does it matter if pocminer runs on a computer with fast (latency & bandwidth wise) access to disks (like directly attached storage VS. USB storage, etc.)?
No one is really doing a hardcore test on setups/configuration.
People will always suggest making a plot for each and every hdd, but opening each individual miner does take up system ram. So, in a situation where someone has 10 hdd's and more, I would suggest a single plot.
As long as the interface is SATA, eSATA or USB 3.0, then it should run fine. I have a test rig where I mine with 2 IDE drive in RAID0 for fun, it's working fine for pool mining.
A fast computer is only important during the plot generation. Then you can migrate the hhd(s) to a slower computer to mine.