First thing to make absolutely sure it that your Pi is not under powered. Very strange things happen if the peripherals that are connected to your Pi do not get enough power. Use a powered USB hub and make sure your Pi is connected w/ a 5 Volt 2 Ampère adapter.
Then check if Linux “sees” the USB disk w/ the command:
(meaning: disk free in human readable format, i.e. GB, MB instead of just bytes). Linux can read almost any file system format: ntfs, ext2/3/4, hfs, fat, fat32, etc. etc. So the Pi is probably able to “handle” it.