A few week ago there was a ransomware called Petya delivered via Dropbox links. The new ransomware is inserted into the master boot record (MBR) of the victim's computer, and the system restarts. On reboot, the hard disk will be encrypted. The solution is described here :
Computer experts from the popular tech support forum BleepingComputer.com confirmed that the technique works, but it requires extracting some data from an affected hard drive: 512 bytes starting at sector 55 (0x37h) with an offset of 0 and an 8-byte nonce from sector 54 (0x36) offset 33 (0x21).
How does the Master File Table encryption/decryption work (For non-expert users)?