Deleting Former RAID Structure on HDD

I just rebuilt my system using a separate "C" drive for the OS and apps plus a second HDD for data. I'm using Windows 7 Pro. The data drive was previously used as half of a mirrored array. The twin drive is on a shelf in the garage. I reformatted the data drive but am getting a message during POST that the system is "not in RAID mode now, there is still RAID structure in the hard disc".

I guess I needed to do more than format the data drive; I'm now thinking I needed to delete and recreate the partition and then format (just one partition, not multiple). In the old days, this might have been handled by FDisk. I assume that there is a better way now.

I'm thinking of copying all the files on the data drive elsewhere, reinitializing (repartitioning) and formatting it from within Windows using "Disc Management". Then I copy the data back and I'm done. Does this seem like a reasonable solution?

Are both systems using Intel RAID controllers?
If so you can change the RAID disk back to normal during boot in the RAID BIOS
I have never had this distroy any data (it says it will but that is for RAID 0)'
It is always a good idea, though to backup what you cannot afford to lose.

Thanks for the help. After reviewing a tutorial, I used DISKPART and it worked perfectly (even down to the time estimate).

I had a similar issue trying to image a 1TB drive that was formerly in a mirrored RAID array.
After trying 3 different disk cleansing/scrubbing programs including Achronis and none worked.
DISKPART worked great even through a USB interface to the drive and did it in minutes!

If I had known this earlier, I could have saved 2 days of fooling around.