did the following steps now disk is not recognized! So I did this for a flashdrive and it worked perfectly then I tried it for an external harddrive and it will not work. After entering:

sudo mount -w -t msdos /dev/disk3s1 /Volumes/MIXTAPE


I get this output:

mount [-dfruvw] [-o options] [-t external_type] special node
       mount [-adfruvw] [-t external_type]
       mount [-dfruvw] special | node


Please help! it will not let me recognize any disk at all when plugged in

Steps followed and worked for flashdrive but failed for other...
From the terminal:

Identify your USB by NAME and IDENTIFIER:

diskutil list


Output is:

/dev/disk3 (external, physical):  
#:                       TYPE NAME                    SIZE    IDENTIFIER  
0:               FDisk_partition_scheme              *2.0 GB     disk3
1:                 DOS_FAT_32 MIXTAPE                 2.0 GB   disk3s1


In this case, NAME=MIXTAPE and the IDENTIFIER=/dev/disk3s1

Now unmount the USB:

sudo diskutil unmount /dev/$IDENTIFIER


Example:

sudo diskutil unmount /dev/disk3s1


Output is:

Volume MIXTAPE on disk3s1 unmounted


Now create the Volume directory - this appears to be the key!

sudo mkdir /Volumes/$NAME


Example:

sudo mkdir /Volumes/MIXTAPE


No output.

Now mount the USB to the Volume:

sudo mount -w -t msdos /dev/disk3s1 /Volumes/$NAME


Example:

sudo mount -w -t msdos /dev/disk3s1 /Volumes/MIXTAPE


No output.

Validate that the USB is now writeable:

touch /Volumes/$NAME/tmp.txt


Example:

touch /Volumes/MIXTAPE/tmp.txt


You should now be able to see that you were able to create the tmp.txt file on your USB in the Finder app or by:

ls -al /Volumes/$NAME


Example:

ls -al /Volumes/MIXTAPE
You could be experiencing a hardware issue.  It could be a problem with your hd enclosure.  But, let's try this. Maybe the format didn't take and you still have the disk formatted for NTFS.  In terminal, run:

 diskutil list


Run this command before and after you attempt to format the drive. Post back the before and after lists.

Do you want to share with Windows?  For now, let's set it up for the mac. I suggest going with the most common setup for an external disk.