Does anyone know what's wrong with my HDD?

Could be bad sectors or a ton of reallcoated sectors. Download and run the Crystal Disk Info in my Signature. Most likely will say caution on the drive or bad. Post back what is listed as yellow and/or red.

Also most "Free" Data recovery software is meant to read from working paritions. Not so much bad drives or lost partitions. If you do plan on buying some if you need it i reccoment Power Data Recovery. I have a Pro License that my work bought a while back and it works magic. If it can't recover the file then it will have to go to seagate for recovery. I've tired other recovery programs and none of them work as good as PDC

Yea that hard drive is pretty toast. a recovery software may be able to get stuff off of there but chances are i wouldn't count on it. at least not with what you could do software wise.

Well the fact that one is Red (It won't show red unless its really bad) If you open the program backup again (Crystal Disk Info) and go to the drive starts again on the right right side of those there should be a HEX number. I can tell you exactly how many sectors are bad. But the fact that it is read means its at least a few hundered if not more.

Now Reallocated Sectors are full on Bad Sectors but they might as well just be. When you have a few hundred or over a thousand of them your hard drive will slow down to a crawl. When i first for my Seagate 1.5TB years ago after a year it would take my PC 15-20 minutes to start up and even then everything was slow. Come to find out i had over 4000 reallocated sectors.

Now as for the other two the Current Pending Sector count are sectors that it is trying to read the data from. It will try and try so man times. If it can read the data then it moves it to the Reallocated Sectors making that number bigger, slowing down the PC more, and then it marks that sector as un useable.

The Uncorrectable Sector count are sectors where it has tried and tried and tried but could not get the data from that sector and gave up and marks it as bad. The thing is getting even 1 bad sector can crash a whole system. if that 1 bad sector is somewhere a needed file for windows to boot up properly is at then windows just doesn't boot up.

This happens for a lot of reasons but it just happens in most cases. Some drive can last 10 years. Some will only last a few months. The larger the hard drive is though (Especially anything over 2Tb in my experiece) They have a higher rate of failing. Only reason why i have a drive larger than 2Tb is because its purely a backup drive. I put maybe 2-300 hours on it a year just to backup my 4Tb Raid 0 of 2TB drives. They could just be old or have defects in it (As my Seagate 1.5 TB 7200.11 drive was highly known for this issue, and every time i got it RMA'ed the returned drive lasted half as long as the previous. I spend enough money in shipping the drive to Seagate to buy a whole new drive).

So it just happens and this is why you should always have a backup. If your data isn't in 3 Different Places you risk losing everything. My company usually reccomeneds a External Hard drive backup that can either be automatic or manual back and a cloud baised service like carbonite. This way, god forbid, the whole place burns up you have carbonite at least.