I just connected my external hard disk to my PC,hard drive not showing, then m opened disk management but machine show unallocated hard drive. Please tell me solution of that
If this is a new drive, you need to go to Disk Management and initialize, partition and format the drive.
If you have been using the drive for a while and it suddenly got unallocated, it may be a good idea to try to recover the partitions with some kind of partition recovery software. Check this case which is similar to yours: http://www.sevenforums.com/hardware-devices/288692-disk-1-unknown-not-initialized-unallocated.html