A few alternatives besides unplugging the drive along with any other usb devices for a fresh detection as A Guy recommended would be trying it on a different port and checking the data cable at both ends to make those are all the way in.
Hopefully the partition table MFT is still good on the volume. If you left the factory Fat 32 volume on it and never reformatted that to NTFS that could be the problem. As for free data recovery programs you wouldn't like one came across lately called Dr.Freeware.
That will recovery "everything" from a drive but assign each file a new number instead of name! 168gb of system files meshed with personal files, exe, ogg, dll, and every other file type from the Windows directory along with files in the user account were mixed together in over 1,000 recover_1, recover_2, recover_3, etc. folders.
The one or two programs fouind that would see each in original form wanted you to spend $70 approximate for the full version to recovery anything! The Dr.Freeware(Fedora Linux based) found everything on the drive where a resized primary was turned into unallocated drive but was able to grab everything when booted live off of the cd you first make up. Dr. Freeware - Data Recovery
Mind you this is strictly a "last resort" type of program that can recover from any partition you select or the entire drive when the partition table is toast. Last resort is the key word for it however.