MacOS – How to mount an encrypted disk image so that it is read-write

You want to run diskutil coreStorage unlockVolume <UUID>, where the UUID is the Logical Volume UUID associated with your encrypted disk, /dev/disk3. You can get the lvUUID by running diskutil cs list in the terminal and looking for the output related to /dev/disk3. The identifier is a long string that would look something like this 'B807C2A0-577F-3DB0-9002-F82B9137696C'.

It will prompt you for a password at that point and if it's correct, unlock the disk.

If you encrypt something once, you can't weaken it by encrypting it another time. You can strengthen it, but not very much that it makes a big difference. Most attacks are brute force password Crack attacks, on both filevault and the disk image, because there is the highest chance for the decrypter to win. So you harden the step of decryption because you encrypt twice, but you don't harden it much. but btw, best anti brute force tactics are using algorithms like PBKDEF2 instead of simple SHA. but that's something only apple can do. and I expect from them to do something like that.