FileVault 2 encrypted drive not accepting unlock password in recovery mode
Just had this happen to me, error -69842 on a FileVault encrypted mid-2013 MacBook Air, and I spent a fair bit of time on it the last few days. I tried Recovery Mode, Target Disk Mode, fsck_hfs, and DiskWarrior with no luck. Disk Utility gave me an error 'Invalid thread record' or something like that. DiskWarrior indicated it was a 'disk hardware failure'. It was a corrupted FileVault volume no doubt.

I was finally able to get at my files by booting up in single-user mode. Hold CommandS while booting. You'll have to pick a user and enter your password, then it'll drop you to command line. The encrypted volume that would otherwise fail to mount with Disk Utility or 'diskutil coreStorage unlockVolume' will be already mounted in your Unix shell. Apparently single-user mode bypasses whatever filesystem check is failing when an error unlocking occurs. Perhaps this is because no fsck is being run, I'm not sure.

I was able to get my files off this way using a connected USB drive. Run mount -uw / &amp;&amp; mkdir /Volumes/usb, then plug the USB in and do ls /dev/disk* to find the name. Mine was /dev/disk2s2, so mount_hfs /dev/disk2s2 /Volumes/usb and I had a way to copy files off. Use mount_msdos instead of mount_hfs if you formatted it for non-Mac use. I selectively copied user folders one thumbstick at a time, but if you have a large enough USB drive attached then just do a tar czpf /Volumes/usb/Users-backup.tgz /Users to get a copy of the entire /Users folder in tar-gzip format.

Recreating the FileVault volume is necessary. I did not find a way to repair it.