You need to fix the bad hdd so it can be mounted. Find out what is wrong with it and either fix it or replace it.
There are plenty of hits if you Google "how do i diagnose a bad hdd in linux" and then "how do i fix a bad hdd in linux".
Possibly you need to restore from backups to avoid data loss if the hdd cannot be fixed.
The easiest way could be to boot a rescue distro from a thumb drive and use the tools to diagnose and possibly fix the hdd. Or try to rescue data.
"how do I create a Linux system rescue thumb drive"
"how do I rescue data from a broken Linux hdd"
"where can I buy data recovery services for linux"
HDDs are most likely to fail when they are brand new and when they are very old.
A HDD that is half a year old is the best. It is like shoes.
oh no! u think a bad HD :(? it's only a week old and it was working fine before I started messing with the union stuff. Ok, i'll try a few things on the HD
Please post the SMART status of disk, perhaps you have luck and the disk is on good state.
It seems the drive has a missing/bad sector.
This should be grounds for a warranty claim. Get a new drive. Talk to however sold you the drive about how to do the claim.
It might be possible to run some tools like chkfs and fix it by avoiding use of the bad sector. And then reformat and have a usable drive. But you will lose all data and the drive might develop more problems. Drive errors often comes in groups. Large groups.
So get a new drive as a replacement and restore your data from backup. And all is good.
If you don't have backups you might want to Google stuff like: "how do i rescue data from my broken Linux drive".
thanks for confirming 
Luckily, only 100 gb or less was used for media.
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