If after system reboot, the external hard disk partition is not automatically mounted then we need to check if the partition is hidden. When I installed or repaired the boot camp driver it sets the D: mounted partition as hidden by default. This will cause Windows to not automatically mount and assign a drive letter for the partition on reboot.

1. Run diskpart in cmd and check automount status.

automount

Automatic mounting of new volumes enabled.

If it is not enabled run automount enable.

2. Next check if the partition has hidden attribute set and if so clear it.

list volume

  Volume ###  Ltr  Label        Fs     Type        Size     Status     Info
  ----------  ---  -----------  -----  ----------  -------  ---------  --------
  Volume 0         EFI          FAT32  Partition    200 MB  Healthy    Hidden
  Volume 1         EFI          FAT32  Partition    300 MB  Healthy    Hidden
  Volume 2     C   BOOTCAMP     NTFS   Partition    256 GB  Healthy    Boot
  Volume 3     D   WinData      NTFS   Partition   1606 GB  Healthy    Hidden
  Volume 4                      FAT32  Partition    200 MB  Healthy    System

select volume 3

Volume 3 is the selected volume.

attributes volume clear hidden

Volume attributes cleared successfully.

list volume

  Volume ###  Ltr  Label        Fs     Type        Size     Status     Info
  ----------  ---  -----------  -----  ----------  -------  ---------  --------
  Volume 0         EFI          FAT32  Partition    200 MB  Healthy    Hidden
  Volume 1         EFI          FAT32  Partition    300 MB  Healthy    Hidden
  Volume 2     C   BOOTCAMP     NTFS   Partition    256 GB  Healthy    Boot
  Volume 3     D   WinData      NTFS   Partition   1606 GB  Healthy
  Volume 4                      FAT32  Partition    200 MB  Healthy    System

exit

Now reboot and the partition will be auto mounted.