I purchased a 16gb USB2 flash drive, dumped about 3gb of music to it, and the drive is not recognized by the head unit.
After some research online, I found other subaru owners having issues too - apparently, it wont support flash drives larger than 8GB.
Though, some have stated that they do work - but, files located past the 8 GB are not recognized.
The Crosstrek manual states NOTHING about a usb storage limit.
So - I am passing this on... and, if anyone is successfully using a USB flash drive larger than 8 GB successfully, please state the name & model of the drive you are using!