Prologue of this story: A very long time ago (2004 to be exact), I decided to create a new PGP / GnuPG key with 4096 bits (due to this talk). However, shortly after that, I had a hardware failure of my hard disc. The home was a dm-crypt partition with xfs. I was able to restore most data, but it seemed the key was lost. I continued to use my old key I had in a backup and the 4096 key was bitrotting on keyservers. And that always annoyed me. In the meantime, I found all private keys of old DOS (2.6.3i) and Windows (5.0) PGP keys I had created in the past and revoked them, but this 4096 key was still there.
I still have the hard disc in question and a couple of dumps I created during the data rescue back then. Today, I decided that I'll have to try restoring that key again. My strategy was not trying to do anything on the filesystem, but only operate within the image. Very likely the data must be there somewhere.
I found a place where I was rather sure that this must be the key. But exporting that piece with dd didn't succeed - looking a bit more at it, it seemed that the beginning was in shape, but at some place there were zeros. I don't know if this is due to the corruption or the fact that the filesystem didn't store the data sequentially at that place - but it didn't matter. I had a look at the file format of PGP keys in RFC 4880. Public keys and private keys are stored pretty similar. Only the beginning (the real "key") part differs, the userid / signatures / rest part is equal. So I was able to extract the private key block (starting with 0x95) with the rest (I just used the place where the first cleartext userid started with my name "Johannes"). What should I say? It worked like a charm. I was able to import my old private key and was able to revoke it. Key 147C5A9F is no longer valid. Great!
P. S.: Next step will be finally creating a new 4096 bit RSA key and abandoning my still-in-use 1024 bit DSA key for good.
In case of future disk/machine failure, you should create backups using a non-disk backup mechanism, ex: paper. In a blog post, I describe two applications that can be used to print machine-readable data to recover: http://blog.eharning.us/2011/04/key-backup-for-paranoid.html
With a paper backup, it is easy to deal with, so long as you have a scanner and have not let the backups degrade beyond recovery.