Or should that read, "Why I am doing this" 
This is a custom specified Alienware built machine, less than 2 years old, which started with 2 SSDs in RAID 0, in addition to two, 1TB standard SATA drives, and Vista 64 bit Ultimate.
From day 1 the SSDs have been a problem with intermittent failure to load Windows, resulting in complete failure to load and RMA back to Alienware twice to resolve the issue. Each time they returned it with a fresh install of Vista, but no indication of what they had to do, despite me asking them to tell me. I suspect the issues were TRIM related, (lack of TRIM on Vista), needing a complete wipe of the SSDs to return the pages to empty. In between the two RMAs I had several phone conversations with their help desk about this, but they were unable to offer any advice. The system is still under warranty but I suspect I have voided that now by discarding the SSDs.
I don't blame Alienware at all. I knew this was an experimental setup when I customised it at the order stage. This type of Desktop is an enthusiast's machine with "Tools Free" access to the inside of the case, but I am certainly not an enthusiast, nor technically minded. So I knew the risks and accepted that. But this was/is a high end system packed with components, quad-core, 12GB RAM, liquid coooled, dual graphic cards, and SSDs, (and a pretty case), and since it is possible this will be my last system I wanted one that would beat the competition initially, and then 'keep up' up with it for a while to come. We already see off-the-shelf systems with dual-core and 8GB of RAM, and now we are seeing SSD boot disks, so I am still ahead of the game, but only just, and the rest of the world will catch up in a couple of years or so. Just guessing of course.
But, and again I am guessing, my SSDs are experimental. They were not common when I purchased this. I know SSD technology has improved to account for the TRIM related problems but mine are older and cannot be firmware upgraded. Intel has a utility that replaces Windows 7 own TRIM controls for better lasting SSD use, but it is too new for my SSDs. I couldn't continue with these SSDs so I have removed them.
That's why I installed Win 7 on one of my 1TB normal drives, but since this is an all SATA machine and since the Win 7 disk is retail, it doesn't have the necessary SATA controllers to manage the SATA drives correctly. You're right that I could have continued as I was, but it really didn't feel right.
Your XP system probably showed the SATA drive as removable because XP didn't have native SATA support. I am guessing again, 
So that's why. This has got to last me for some time and I need it to be right.
I can't remember the last time we had a power cut here in Derby. Well over two years ago.
It has all been hard work for me, and always frustrating, but again I am not complaining, (well, not out loud), and I am learning as I go.
Now to try and reinstall the Intel SATA technology again!
Mark