Second western digital HD not mounting

I know this isn't a new question but I've been trolling forums for days without any clarity. I'm a professional photographer who cannot manage his photos well. Now its time to get serious. I have 2 Western Digital My Book (# 0x1102) for Mac 1Tb HD. Im running on an early 2008 24in iMac with OS X 10.7.4. What I want to do is set up a RAID so that one HD holds my files for Lightroom and the other has them automatically copied. And eventually have third external HD to manage time machine.

What I have now is 1 HD (WD 1Tb) with all my LR photos and time machine back ups. But the only back ups I have for my photos are ones I inconsistly make on CD (no good). And that HD is full.

The problem: When I plug the 2 HD in, 1 via firewire 400 the other via 6 pin (HD) to 9 pin (Mac) cable, so 400 on the HD 800 on the Mac only 1 HD mounts. The one that is full (HD 1) mounts over the nearly empty one (HD2). To get HD2 to mount I have to unplug both and replug HD2.

I've tried:

rebooting (mac and HD)

shutting down, uplugging the Mac, then replugging the HDs.

plugging the HD into separate power outlets

switching the ports on the HD and on the Mac

Swiching cables from one HD to the other and trying new cables

configuring the finder side bar to include all external HD

Also, any suggestions on how to get the data from HD1 backed-up for when I need to wipe them to set up the RAID?

I'm also open to suggestions on how to do this more simply.

Thanks-