So I reinstalled Windows 7 and apparently something went wrong: during the install I had two drives connected (drive A & B), I selected drive A for Windows to be installed to. Later I noticed that I can't boot with just drive A connected - I also need drive B for it to work (neither does it work with just drive B connected, I need to connect both drives and select drive A as boot first device).
Now here's my problem: before I noticed the problem I connected a third drive (C), disconnected drive B and tried to boot from drive A.
Drive C had two partitions: 100GB of unencrypted unimportant data and about 2,9TB of encrypted (AES with TrueCrypt) important data.
Now it has these two partitions: about 130MB of "Microsoft Reserved Partition" and about 3TB of unallocated data.
I really need that data back. I don't care how much effort, money or time that may require.
(And before you tell me: I had a backup of most data on drive B which doesn't work anymore either because of the same problem - it was a system encrypted drive with Win7)
Currently my next step would be to try out TestCrypt. Any suggestions are welcome.