is ur drive FAT32 type of NTFS??
If ur partition is NTFS type than read here:
Do you large number of files...they may have cluster the dive.So,do cleaning work with some good cleaner.
you can find some system cleaner here:
For error U are getting that " cannot acces drives ":
There may be some device/program that is reading/writing to
the drive. Some anti-virus applications do this.USe UNLOCKER to find what program are running .
Download unlocker from here: