I totally agree with Shane... and this error has caused because either your drive is formatted using FAT32 file system or the structure of your hard disk is damaged which Mac OS X is unable to recognize due to corrupt file system.
In order to resolve this issue change the file system of your hard drive from FAT32 to HFS. However, after changing the file system, drive can not be used with Windows computer. And if the structure or file system of your hard drive is damaged, then you need to format the drive. But, doing so will erase all the stuffs in your hard drive. So, in order to recover them back either you can use available backup or commercial
Stellar Phoenix Mac recovery software. This will surely resolve your issue...
All the best!
