Legacy File Vault

You don't say what happens when you try to turn it off, but usually it's because there isn't enough free space on the startup volume to hold both encrypted and decrypted data at the same time. Move most of the files in your home folder to an external hard drive temporarily, then log out and log back in. You should now be able to deactivate legacy FV. Move the files back to your home folder.

I've had exactly this problem for over a year now, since Lion.

Neither of these things worked for me, but I still think they are worth trying because the alternative is such a bind:

1. You could try repairing the disk.

2. You could try cloning to a MUCH larger disk and then decrypting on the clone (I know you said you got over the filesize thing, but I'm not wholly convinced of the accuracy of FV's error messages).

The way I solved this was 'the long way round'. While booted into your FV-enabled account, you can manually copy off your data to an external disk and it won't be encrypted. You then need to reinstall your OS and manually load your data and apps back on. It's a pain, but its the only way I know how to do it.

(I still maintain the original legacy FV enabled disk as a backup, ironically enough!)

Back up all files in your home folder -- not the FileVault sparsebundle itself -- to at least two separate storage devices, if you haven't already done so. You can't do this with Time Machine. Then create and log into a temporary administrator account, and delete the sparsebundle. You should then be able to deactivate FileVault. Restore from one of the backups.

Linc Davis wrote:

Then create and log into a temporary administrator account, and delete the sparsebundle. You should then be able to deactivate FileVault.

Nice twist. Didn't think of that.