I have plugged external hard drive. It does not show in Normal Mode. I can see its Type Name and Identifier thru terminal.
- I want to find the file system of the external hard drive.
- I want to format the disk.
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First, it is important to be aware that technically Drives are not formatted, Volumes are. One drive can have multiple volumes, even if in practice most users just fill the whole drive with a single volume.
Normally the drive's volume(s) should appear on the desktop when you plug it in. If that were the case, you could simply hit Command+I with a volume highlighted and see its Format listed in the General section of the Info window that pops up.
Since the drive is not appearing in that case, you will need to run Disk Utility. If the drive is available there, you will see an icon and name for the drive on the left, and then below that an icon and name for each volume on the drive. Click on the volume (the indented one), not the drive, and at the bottom of the window you will see a Format field reporting the format of that volume.
Make sure that you click on the Volume, not the Drive, because as I explained at the beginning there is no format on the Drive itself.
From there you can easily reformat the drive by clicking the Drive (not the Volume) and selecting the Partition tab to the right. That brings up a tab where you can delete existing partitions, create new ones, and format them. You can treat Partition as a synonym for Volume (they're not exactly the same thing, but for your purposes they are).
All that being said, if your drive/volume is not appearing when you plug it in, it is likely that either the drive is crashed or the volume is corrupted.
In the latter case, you may see the Drive icon but not the Volume icon in Disk Utility, and you can try erasing the drive from the Erase tab and reformatting.
If you don't see either icon in Disk Utility, the drive is crashed/broken and should be replaced.