OK .. Linux can read/write to Windows formatted (NTFS, FAT/FAT32) hard drives out-of-the box, but it needs a few things installing to read/write Mac formatted (HFS/HFS+) hard drives.
So just in case it's Mac formatted, let's install those tools first .. open a terminal (Ctrl++Alt+T) and run:
sudo apt-get install hfsplus hfsutils hfsprogs
Now let's see how the drive is formatted .. With the USB HDD attached, can you send the output from:
FYI, after installing those tools, Linux will be able to read and write to a Mac drive if it's formatted as HFS .. but if it's formatted HFS+ it will be read-only unless you attach it to a Mac and use the Mac "Disk Utility" to turn off journalling.
If there's nothing on the drive you need, and its going to be used exclusively for Linux, let me know as we can just format it EXT4 or NTFS (or possibly HFS) depending on the backup strategy you eventually decide on, but we'll get to that in a bit
Will the drive be used exclusively for Linux ? .. or will it also be used for Windows and/or OS X ?