There are some raid controllers out there that will let you grow a volume (add a disk or migrate to/from raid 1/5/10/50). You could build a raid 1 volume from the one disk and then expand it to a raid 5. However, if anything went wrong all your data would likely be lost (or at least potentially). If you can, talk the host into it simply bringing up another server temporarily with the raid configuration you want and transfer the data over via crossover cable. It's a lot safer and less error prone.
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