I've just had my new USB unit fitted by my dealer, but it's a bit of a mixed bag.
I had a USB disk in with only a small amount of songs (around 500 I think), and when I turned the car off for a while and turned it back on it started where it had stopped (after around 30 seconds). In my enthusiasm, I thought things were fixed, so copied all the rest of my music onto the USB (around 6000 tracks). Next time I got back into the car it did start from the same track it stopped at previously but the screen wasn't fully updated, when it got to the end of that track, rather than playing the next in sequence from that album / folder, it reverted to the first track on the USB?
I think it seems to be an issue with the disk still being indexed again so whilst it's remembered what the last track was to be played, it hasn't yet created a full index so doesn't know what the next track should be so reverts to the usual nonsense of just playing the first one on the disk. If I try and go into the artist / album lists during this time they are blank so that's what's leading me to this conclusion, but if I leave it longer they do all appear.
I've tried thinning out the amount of tracks and give the car a longer drive to see if can successfully index the whole disk and then play consistently, but the new unit is a bit less than I was hoping for. I can only imagine that if it resumes at the end of the track you were last listening to it will revert to the very beginning of the disk no matter how few tracks are on the disk.
I'm not sure if a USB 3.0 disk would work any better as I know they are faster, but for what it's worth I'm testing this with a 32Gb USB2 drive.