Decreasing hard drive sync time with Unison

I'm using Unison to synchronize my music collection to a few different sources, namely a couple computers over SSH and a hard drive.

One thing I've noticed is that SSH updates are incredibly fast in comparison to hard drive syncs, possibly because rsync (or whatever's running under the hood) is only moving what's changed. Add that to the fact that I can run multiple SSH copy actions at once and copying to a local USB hard drive is left in the dust, as it only copies one file at a time and overwrites the entire file every time.

When changing the encoding of ID3v1 tags (for compatibility tests), my entire ~5000 file music library changes needs to be backed up to the other computers and hard drive.

Is there any way for me to:

1. increase the number of simultaneous copies; and
2. only copying the parts of the files that have changed?

Here's my Unison profile:

# Unison preferences
batch = false
confirmbigdel = true
copymax = 10
logfile = .unison/unison-music.log
maxthreads = 30
perms = 0
sortbysize = true

root = /home/me/Music
root = /media/truecrypt1/media/music