Lost Vista Ultimate on Boot-up

Running a workstation w/ 6 hard drives. My first O/S on separate HD is Win
2K Pro; 3 yrs. later, I added XP Pro on separate HD; last yr. added Vista
Ult. on a separate HD. All worked well. Got to choose which O/S or HD to
boot after the SCSI initialization every time I powered-up. Foolish me: I
decided to load one of the 3 idle HDs w/ XP Pro so I could use it for gaming
exclusively. Now when I power up I STILL get 3 choice, but XP Pro (new), XP
Pro (old), and Win2k (old), BUT nowhere is my Vista Ult. displayed as an
option to boot into. What happened and how do I get that bootable choice to
display?
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Thomas Peter v B