Changing letter designation of Mac OS X partition via Windows can lead to data loss
You should not change or remove the letter designation of a Mac OS X partition while running Microsoft Windows, such as via Boot Camp.
Tools such as Windows Disk Management may be capable of changing or removing the letter designation, but this can cause the Mac OS X volume and the data contained on it to become inaccessible.
If the drive designation letter has already been altered or removed, third-party utilities may allow the recovery of the Mac OS X partition by restoring its GUID. The GUID is:
48465300-0000-11AA-AA11-00306543ECAC
Apple does not support or endorse specific third-party disk utilities, but they may allow for recovery of the partition and data.
Important: Information about products not manufactured by Apple is provided for information purposes only and does not constitute Apple’s recommendation or endorsement. Please contact the vendor for additional information.