Get rid of using Flac and any Lossless type all together, start using the mentioned 128 compressor people have talked about before.
1.5tb will take awhile but the payoff is well worth it in the end, small, universal, and a sound quality not to be messed with.
I had to redo my entire collection of 4,500+ lossless in two weeks flat for my business (had no choice between formats..only mp3), it got done but needless to say i didn't leave the house much. the major time killer is tagging all the tracks out...
hope you at least tried the compressor mentioned and compared the two formats, if not then do it when you get a chance. it killed apple lossless head to head everytime on every set i tried,,,and that was a lot of listening!