I found the bad drive--or cable lead. The Adaptec 3405 BIOS just up and decided that one should be grayed out. It recognized it, but for some reason wouldn't allow initialization or let it be included in an array. On the other hand, the drive didn't mind being verified or formatted--with no errors. There was no feedback that I could find on why said drive was shunned. I built/verified the array with three drives and no problems.
I should have four more drives from Seagate tomorrow so I'll use one of those and try again to make sure it's not the cable.

Update: I received the four new drives from Seagate today, replaced the one the Adaptec 3405 deemed bad and the RAID 5 build/verify routine went off without a hitch. Unfortunately, the 3405 with the new BIOS I flashed in over the weekend now isn't playing nicely with the D5400XS motherboard--freezing the latter's BIOS when I try to alter the boot order. I'm flashing back to the older 3405 BIOS tonight which behaved better.
Notice in the photo below that I've attached two Vantec Iceberq (don't ask me why the Q) hard drive coolers. The fan in front of the hard drive cage just wasn't cutting it on its own. Now the drives run acceptably cool, that is, only slightly warm to the touch.

I like these so much, I'm going to grab a couple for myself as well as one more for the top drive which runs a little warmer than the rest since it's the only one not directly affected by the increased airflow. Anyone remember my article on the Carnegie Mellon and Google reports that hard drives don't last nearly as long as the manufacturers would like you to think? Well, they claimed heat wasn't that big an issue, but the ranges they were talking about were far less than what you find in many PCs, let alone one with four Seagate Cheetahs spinning in close proximity. Cooler IS better!

Thanks to Lonny and Tiger Direct for the cooling supplies!

Cheers, Jon