Cannot read external hard drive after installing Linux?
Well, if you had data already on the drive, it's gone. You could use a data recover, or ship it off somewhere. But, I'll just go into formatting the drive.

A. Sorta. Generally Linux uses a diffrent type of format for drives (ext4, and a couple others) so it could be that it did work, and your Mac just can't read it.
B. If it really did get messed up (you can't boot it into it) then :


Go to Disk Utility.
Click on your drive.
Click on the "Erase" tab.
If you'd like to, enter a name for the drive. Or, just keep it as Untiled if you'd like to try Linux again.
Select a format type, if you'd like to. Fat works best for cross-device compatibility.
Click the "Erase" button.
Get some coffee, or water, depending on the size of your drive.