Apparently there is a big secret issue here that nobody has figured out. I've read through about 20 forums trying to find a solution or at least, to find out what the hell the problem is.... And not even Microsoft has posted anything about it to my knowledge. I think I figured it out though.
Is your new PC running Windows 8.1? or Windows 8? And your old PC was running Vista or Windows 7?
First off, don't worry... there's nothing wrong with your disk. It's not faulty as some people suggested. It's a simple problem, but the solution is a pain.
I built a new Desktop recently, and I installed Windows 8.1. Then I took 4 external hard disks connected by USB that I was using with my old laptop, (which was running windows 7) and cracked open the cases and installed the hard disks directly into the desktop with SATA cables etc. After I was finished installing everything, I booted up the PC for the first time, saw that all the hard disks were detected by the BIOS fine, and proceeded to install Windows 8.1 etc. I installed Windows on a SSD drive, which is not related to this story. My external drives were only used for storage. So my situation might be a bit different than yours, if you're trying to install Windows on the drive that is having issues...
Anyway, after Win 8.1 was installed, to my dismay, I discovered that ONLY ONE of the external drives I installed showed up under "My PC" as being accessible. The OTHER 3 showed up as unformatted, and when I tried to open them, it told me I needed to format the drives before I could use them. That's when I started scouring the forums for a solution. Some people suggested teskdisk.... which only helps your recover data. It doesn't actually fix the hard drives issue. So, here I had 3 drives I couldn't access, but one I could... with no idea why.
Anyway, I used testdisk, and I could see all my files were there on the 3 trouble drives. But the problem was I had nowhere to copy the files to. My other drives were all full.
So I purchased a new 4TB drive, so I could retrieve all my data easily which is what I'm doing now. I'm copying files as I write this... But there's more to this story...
After I got the new drive and installed it... It asked me to format... but it gave me an important choice. It asked me if I wanted to use MBR or GPT partition tables.... I had never heard of GPT before. And the funny thing was, I checked and I noticed in the drives information for the one drive that DID work before, that the partition table was "GPT"! I had never even heard of GPT before. So, I did a little more research and discovered that GPT is the new partition system, and MBR is being replaced. Nobody knows about this apparently! Or poeple know about it, but have no idea that MBR disks DON'T WORK IN WINDOWS 8. Or at least, some types of MBR formats don't work... not sure if they are all the same. So of course I checked, and found that the 3 drives that didn't work were all MBR!
So yeah... There's a problem with transferring hard disks between PC's running older versions of windows and Windows 8.1 with relation to the partition tables. The problem seems to be with the MBR (Master Boot Record) partition system vs the new partition system called GPT (GUID Partition Table). In Windows 8, hard disks that have the old MBR system and are simply connected with that type of partition will come up as unformatted and unavailable for use. It's not till you convert it to GPT in the Computer Management screen that you can use the hard drive with Windows 8. However, if you have data on the drive you want to keep, this will probably cause some problems.
So, as other people suggested, you will need another blank hard disk (formatted with GPT of course) and then use a utility like Testdisk to copy the data over to the new disk. Then format the old drive using GPT instead of MBR, and then it will work, and you can copy your data back. This process assumes you have a spare hard disk. If you don't... you may have to buy one. I haven't found a way to convert the disk to GPT without formatting... there might be a way... but I've already purchased a new drive, so I'm going with the simple solution and safest solution. If you convert to GPT, at the very least, you are forced to do a quick format.... which doesn't actually erase the data... It just marks all the data clusters as being empty or over-writable, but the original data is still there. If you do a full format, you will actually clean the data clusters completely, and your data will truly be gone… in the sense that only a team of engineers would be able to get it back after that…
Therefore, if you do a quick format, a data recovery utility might be able to recover the data but it might be messed up and useless... it depends on the utility. I’ve seen utilities that rename the files and give you big blobs of useless data with no file extensions etc. and I’ve seen utilities that will give you your files back exactly as they were before they disappeared. However, I think the safest bet is to backup the data to another drive using something like testdisk. It's a huge pain in the ass.... but if your data is important to you, you don't have much choice. And I just did it myself and it worked. All of my data is there.
Unfortunately, the programmers at Microsoft never thought of this issue when changing the partition table system, and left us all in the dark on it. They never even mention this issue in their knowledge base... gawd I hate those pricks... how could they not consider issues like this? And why doesn't Windows recognize the issue and TELL us the problem when we try to open the “unformatted” hard disks. No, instead it gives us a simple vague message, telling us the drive is unformatted (even when we KNOW IT IS FORMATTED) and tells us to reformat the drive...
Anyway, hope that information clears things up for some people.
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