Using an external USB hard drive
Just wanted to share my experience. I've had my 2012 Focus for about 1 month. Speech commands worked fine for like the first two weeks. Then, after one of my many trips with the HD back into the house to load more music, I just got the "speech control not available with this device" message. I'm using a bus-powered USB 3.0 1TB Seagate GoAgent Flex 2.5" external hard drive, formatted FAT32.
Within that last two weeks, speech commands have worked exactly one time. I decided, today, to call Sync support, and the lady told me that there is a limit of 20,000 characters for music speech commands - an absolutely ridiculous limitation, IMO. This includes the artist, track, album and genre of all tracks (all the searchable fields), combined. I somewhat verified that today by maxing out a 4GB USB stick (probably less than 500 songs) and trying it instead of the hard drive - speech commands worked almost immediately.
I still love the car, but it looks like I'm gonna be figuring out something else for music.