Windows – How to get rid of this old hard drive in the machine

I bought it about 5 years ago and it came bundled with Windows Vista.

When Windows 7 launched, I waited a bit then realized it would rock with an SSD.

So I bought an SSD and added it as my primary SATA drive (set in BIOS) then installed Windows 7 from the CD I bought.

I didn't wipe my old HD and kept it as D:\ cos I needed all the data off it.
I thought this was a good idea at the time, but during boot now I get two options for boot

1. Windows 7 on C:\

2. Windows Vista on D:\

Weird, but I lived with it for as everything was sweet for 5 years…

So now years later I've finally got around to migrating all the data off it and onto a NAS… and I wanted to get rid of the old HDD.

But when I just unplug the old HDD (power and data)… my BIOS complains it has no drive to boot off of.

What's the deal?

How do I delete this old HDD?

I'm afraid of just formatting it cos if it has a boot sector that's needed I won't be able to boot this machine (which I need for work).

Does anyone have any ideas or advice?

UPDATE: I discovered my HDs weren't in the correct boot order. I corrected the boot order of the hard drives in BIOS, but still get the same problem.

Also, thejourneymans suggestion didn't work after a corrected boot order.