I've been mucking around with this for a little while, and am "almost" finished with it.
In short, the install consists of:
Nexus 7 2013 2nd gen, rooted, Timur's kernel, and a few of the usual things installed.
USB 3.0 powered hub. (Both tablet and hub are powered by a second cigarette lighter socket/dual USB lighter socket thing, wired off - but behind - the cigarette lighter below the stereo).
FSL-21 connected into the back of the McIntosh head unit, and a Behringer USB DAC connecting the tablet to the FSL2-21 (and hence the stereo).
I've also connected a 200Gb 2.5" laptop SSD to the hub (it's in one of those transparent cases which provide a USB-SATA "bridge". The SSD is ostensibly to hold a music library of ~150Gb. There's also a 16Gb USB stick, ostensibly for maps for the TomTom app.
Both USB drivers are "seen" in ES Explorer Pro. However, the SSD causes errors, with the message "this drive is not suitable for this device, do you wish to format" appearing in notifications. I'd initially formatted it as NTFS and copied the music on to it (on a Win7 laptop). The first time the error got thrown, I took it out of the car, re-formatted it (as exFAT), and re-copied the music. Re-installed in car, and the same issue occurs.
I must be doing something wrong...but what?