I have a 2011 MacBook Pro Intel running Mountain Lion. I have a 2tb Seagate External Drive with 3 partitions: all three were set up the same way with different passwords as a person who used to live here was burning DVD's from my digital backups I felt the need to do so. I was having no issues with this for the first six months I had the drive set up like this but about two months ago this changed. I can no longer mount one of the partitions. It will not accept the password I KNOW goes to it. All I can figure is that, some how, the system encryption key must have been damaged. The only other pieces of Info I can give you is this. First, I didn't use firevault for this but Disk Utl. Second, at times, I can mount one of the other drives and even though I get the "shaking window" as though I typed in the wrong pw it still mounts the drive. Odd.
Same problem here. Read and on and I'll tell you what I did to solve this problem:
I do Time Machine to an encrypted external Hard Disk. I stored the password to the disk in the default Keychain, and the system mounted the disk automatically after plugging it in. But suddenly, last morning, after I plugged in the disk, it prompted me for a password (it never did, since it was set to remember the password). In that dialogue, type a wrong password and the dialog does the wrong-password, shake animation, and it prompts for a password again; but type the correct password and the dialog freezes, like this:
The solution I have found to work is as follows:
And now, my Time Machine drive works normally again:
So, launch Disk Utility, then Verify and Repair your faulty disk as needed. This worked in my case.
Andres's answer is good (Thank you!), and if the first attempt to repair doesn't work, you might have to run it a few times:
and yes then it did reappear from 'greyed' to 'colour' disk on the side panel.
For me to see it mounted, I had to unmount it, pull the USB out and replug it back in.