|"...in windows explorer and my computer.."|
Almost no one who starts a Topic on this site mentions Windows Explorer . I prefer it to My Computer myself.
If it wasn't you who software partitioned (using the NTFS) and formatted the drive, the size of the drive letter's partition in Windows in My Computer and Windows Explorer is not necessarily the size of the hard drive's space.
In Disk Management, if the total drive size is 931GB, it should show you either that part of the drive space is un-allocated, or that the physical drive has more than one partition on it (more than one drive letter).
If it shows only one partition and there is no unallocated space on it in Disk Management, run chkdsk .
If you were accessing the drive, access something else.
All Programs - Accessories - RIGHT click on Command Prompt, choose Run as Administrator.
type: chkdsk /f (the drive letter followed by a colon) , press Enter.
E.g. if the drive letter is G
chkdsk /f G:, press Enter.
If Windows tells you something is accessing it and should you allow it to un-mount it, answer yes, press Enter.