## list all drives and find /dev/sdXY for the external
sudo lsblk -o NAME,FSTYPE,SIZE,MOUNTPOINT,LABEL
## Install hfsprogs to read HFS+ (apple's file system)
sudo apt-get install hfsprogs
## mount
sudo mount -t hfsplus -o force,rw /dev/sdXY /media/external
## or
sudo mount -t exfat -o force,rw /dev/sdXY /media/external
## if the drive was improperly unmounted or a little corrupted
# sudo fsck.hfsplus -f /dev/sdXY
## to unmount
# sudo umount -f /media/external
## to edit the partitions
# fdisk /dev/sda
## to format ntfs:
# sudo mkfs.ntfs --fast /dev/sdg2