El Capitan Fat32 partition problem
If you enter diskutil list / in Terminal only partitions of the boot device are visible.

The proper command to enter is just diskutil list.

With the disk identifier diskX found above try to verify the disk and the volume:

diskutil verifyDisk /dev/diskX
diskutil verifyVolume /dev/diskXsY


If verification was successful try to make a mountpoint:

sudo mkdir /Volumes/EXT_HD_FAT32


and try to mount the partition there:

sudo mount -t msdos /dev/diskXsY /Volumes/EXT_HD_FAT32/