Do not perform the conversion you will LOOSE your data.
It sounds like the disk has been converted to a Dynamic disk under an OS such as XP Pro / Win 2k3 and you are attempting to open the disk on either a laptop or a XP Home PC. (Both do not support Dynamic Disks) This situation will mark the drive as forign and offer you to convert it to a basic disk.
Agin the conversion will blank the drive.