Hi mani01 and welcome to LQ.
It will help to know your level of experience with Linux, drives under Linux, and the command line.
First: What Linux distribution and version do you have?
Second: What are the system specifications for your computer?
Third: This external hdd is a magnetic drive? solid state drive? other type? Is it attached over USB?
Fourth: What do you know about this drive? Is it a Windows drive? A brand new drive? Or something else?
What are you trying now? Are you examining the drives found using the command line? Have you checked the system log to see how it gets detected when you plug it in?
What parts of these questions are blocking you where you need more information?
Some general mount information:
- Typically Linux distributions can support automatic mounting, but there are many varieties which will not. Further, if the drive has no file system at all, then it will not be mounted
- The mount command, as done using the command line, also known as the terminal, requires root privileges. Typically users use the sudo command modifier to make that happen.
- The type of file system is important to know to aid the mount command with it's actions, such as ext2, ext4, fat32, or other type.
- The discovered drive letter is also important. For instance, Linux typically assigns resources under the /dev file system and hard drives it usually makes them /dev/sd<letter> where the letter starts with a and continues to z. I myself do not now what comes beyond z, if that's ever needed and probably have only ever seen enough complexity to go as high as maybe /dev/sdh
Giving us some more information and helping us understand your experience level will be helpful.