My D drive is not accessible, instead it gives me a message to reformat it. However, the properties show me the allocated and free space. What are the causes and solutions?
Filesystem is destroyed on impossible to create. It used to happen to me when they sold me a 16Gb pen drive and it actually was a 2Gb firmware hacked one. So copying file into it and formatting were a real mess!
Your windows system file policies is corrupted somehow, so use a restore point and rollback to a previous state where everything was fine. Tape f8 and do not go to safe mode instead use the repair windows function on the top.
Is this a drive that you have used before? If not, it could have been partitioned, but not formatted, or not formatted properly. If you have used it before on this computer successfully, the formatting could have been corrupted.
For a solution, formatting it would probably fix it. If you have data on it that needs to be saved first, using a LiveCD version of a Linux distribution, such as Ubuntu might allow you to rescue files before doing the formatting.
It could be that the drive is corrupt, corrupt NTFS, happens all the time with older drives. If this is the 3rd or more time you've reformatted and it is a new HDD, then you should do a lowlevel format in order to check for bad blocks.