user4237: In short, 

I bought a few BTC years ago when they were very cheap.
I reformatted my HDD and reinstalled the OS.
I have used PhotoRec and
http://www.cyplo.net/2012/04/01/bitcoin-wallet-recovery-photorec/ to
recover 3 .dat files. Searching only for the ''detection signature''
of a Bitcoin wallet.dat

And the confusing part (if reformatting a hard drive containing bitcoins was not confusing enough!) is that the recovered .dat files are 7GB , 13GB and 8 kb.
Obviously, trying to use the recovered wallet.dat with Bitcoin-qt failed. 
Chances are that these .dat are not wallets but I though this would be worth a shot considering how massively more valuable BTC are compared to 3 years ago. 
Anything I can do with these recovered files?
Thank you!

user10329: Checkout pywallet. Its a python script that may be able to help you out by recovering your private key. I've had similar issues with Multibit in the past. Is your wallet encrypted?
https://github.com/jackjack-jj/pywallet

user12742: 
Install Python 2.7 and download pywallet to your c:\ drive. 
Copy your corrupt wallet.dat file to c:\ 
Open a Command Prompt and type:
C:\>pywallet.py --dumpwallet --datadir c:\  --wallet=wallet.dat --recover --recov_device=c:\wallet.dat --recov_size=416Gio --recov_outputdir=c:\

It should extract all the keys to a new wallet on c:\ 
Copy the new wallet.dat to the c:\users\USERNAME\Appdata\Roaming\Bitcoin folder 
rename it wallet.dat. 
Start up Bitcoin-QT with the --rescan switch. 
It should take a while, but eventually it will start up and your coins are back.


4237: <p>In short, </p>

<ul>
<li>I bought a few BTC years ago when they were very cheap.</li>
<li>I reformatted my HDD and reinstalled the OS.</li>
<li>I have used PhotoRec and
<a href="http://www.cyplo.net/2012/04/01/bitcoin-wallet-recovery-photorec/" rel="nofollow">http://www.cyplo.net/2012/04/01/bitcoin-wallet-recovery-photorec/</a> to
recover 3 .dat files. Searching only for the ''detection signature''
of a Bitcoin wallet.dat</li>
</ul>

<p>And the confusing part (if reformatting a hard drive containing bitcoins was not confusing enough!) is that the recovered .dat files are 7GB , 13GB and 8 kb.</p>

<p>Obviously, trying to use the recovered wallet.dat with Bitcoin-qt failed. </p>

<p>Chances are that these .dat are not wallets but I though this would be worth a shot considering how massively more valuable BTC are compared to 3 years ago. </p>

<p>Anything I can do with these recovered files?</p>

<p>Thank you!</p>
 10329: <p>Checkout pywallet. Its a python script that may be able to help you out by recovering your private key. I've had similar issues with Multibit in the past. Is your wallet encrypted?</p>

<p><a href="https://github.com/jackjack-jj/pywallet" rel="nofollow">https://github.com/jackjack-jj/pywallet</a></p>
 12742: <ul>
<li>Install Python 2.7 and download pywallet to your <code>c:\</code> drive. </li>
<li>Copy your corrupt wallet.dat file to <code>c:\</code> </li>
<li><p>Open a Command Prompt and type:</p>

<pre><code>C:\&gt;pywallet.py --dumpwallet --datadir c:\  --wallet=wallet.dat --recover --recov_device=c:\wallet.dat --recov_size=416Gio --recov_outputdir=c:\
</code></pre>

<p>It should extract all the keys to a new wallet on <code>c:\</code> 
Copy the new wallet.dat to the <code>c:\users\USERNAME\Appdata\Roaming\Bitcoin</code> folder </p></li>
<li><p>rename it wallet.dat. </p></li>
<li><p>Start up Bitcoin-QT with the <code>--rescan</code> switch. 
It should take a while, but eventually it will start up and your coins are back.</p></li>
</ul>
