When viewing a windows client using the VMware Plugin, a drive is missing / not listed in the backup selection box.
Check the type of partition on the missing / not listed drive. If the drive is using GPT (Guild partition table), it is not supported at this time. GPT is unsupported, despite its appearance in the Virtual Disk API Programming Guide as a volume type, GPT (GUID partition table) volumes are not supported by VixMntapi.
The VixMntapi from VMware does support the following Windows file system: FAT, FAT32, and NTFS, however it does not support GPT at this time.
For more information about the VixMntapi from VMware and GPT see theVMware VDDK release notes at: http://www.vmware.com/support/developer/vddk/VDDK-500-ReleaseNotes.html
To be able to backup the drive that is missing / not listed in the backup selection box that is using GPT, you will need to reformat the drive to a supported format such as NTFS.