Drive D: disappears REPEATEDLY

I have a Sony VAIO laptop running Windows 7 Home Premium 64-bit with a single 500 GB hard drive. A year ago, when the computer was new, I used a free partition manager program (Partition Wizard maybe?) to allocate about 60 GB for drive C: which holds Windows 7 and all of my software apps. I allocated the rest of the free space to D: for all of my data files. Sony also has a 13GB recovery partition and a 100 MB System Reserved partition which preceed C: and D: on the hard drive.

Three days ago, when I booted the computer for the day, drive D: was gone. Disk Management showed free, unallocated space equal to the size of my missing drive D:. Using Sony live chat support, the technician used Disk Management to allocate the 400 GB of free space to D: and formatted the partition. I then restored my missing data from a backup.

The next day I booted my computer and drive D: was gone again. Disk Management again shows free, unallocated space equal to the size of my missing drive D:.

I have no third-party disk management software installed. Recent changes to my system include:
1) SmartSync Software recently downloaded a new version of SmartSync Pro (version 4.0) to my hard drive.
2) I ran Disk Cleanup on drive C:

Do you know what would cause drive D: to repeatedly disappear on boot up and how to fix the problem?
Thanks for your help!

I searched the threads for awhile. Although there are many for "Drive D: disappears", none of them are like my situation. It appears to be unique--a first for my particular situation and symptoms.

I would really appreciate some help! If you need more information, let me know.
Thanks!

Have you run the drive manufacturer's disk utility?

What does SMART report?

Have you done a system file check (sfc /scannow)?

Do you see any errors in Device Manager?

When did you last do a complete virus/malware scan?

If a partition keeps disappearing, I would make sure all of the data on there was backed up and proceed to run a Disk Check on the drive. It's possible that the drive is failing, or that you've contracted some malware that is wreaking some havoc with your system.