You need to mount it. First, create a folder somewhere. For example, I'll say it is /home/disk and assume that you have a FAT32 disk. Here's how to mount the disk:
sudo mount -t vfat disk_location /home/disk
Now the hard drive is mounted and you should see it. If you have an NTFS disk you need to replace "vfat" with "ntfs." And as to device locations, USB and SATA disks are designated /dev/sd*, where /dev/sda is the first SATA/USB device, /dev/sdb is the second, dev/sdc the third and so forth. That is the disk's location, but you must mount a specific partition to read data. If you have only one partition (just formatted the disk and that's all), then you mount /dev/sd*1.
So if you have an ATA internal hard drive and that USB external with only one FAT32 partiton, you would type in
sudo mount -t vfat /dev/sda1 /home/disk.