Can the default playback mode be adjusted? I have a USB drive plugged in but every time I start the system, it plays the radio. I need to manually adjust it to the USB drive each time.
The standard operation is to continue to play the last selected source. There are a couple reasons it would not work correctly. One is that you are using a crap Chinese knock off USB hub and not an OEM one. You also could be using a garbage USB thumb drive that has a very slow response time.
If the APIM detects any delay it will default to FM. Both of the above mentioned could cause this. There is no way to adjust this in the AB or other settings.
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The only work around I know for this is to have USB as your selected source when the car is shutdown.
Then use remote start to start up the car before you ever get in, this give the APIM time to wake up and power up the USB port before you hit the start button.
The other solution (if you can call it that) is to delay starting the car for about 10 seconds or so when you enter it to allow for the sync to start ‘rolling/wake up’ as mentioned earlier.
I find that mine will then revert to USB if left at that setting at last shutdown instead of going to FM radio.
In my case this ‘bug’ commenced with the download of version 3.4.22251 and continued through to 3.4.23188 but it is no biggie and I don’t worry about it.
My USB hub is the original OEM and the thumb drive may not be ideal but I am hesitant to try and transfer over 2,000 songs to a better brand thumb drive which could solve this issue as indicated earlier by @bill32399 
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You can also use an SSD. I have a Transcend 256GB ESD400 USB 3.0 Portable Solid State Drive with about 8500 songs, works well, very fast.
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3.4.21265 actually isn’t as bad with the “bug”
On my personal vehicles that are older than 2019 I’ve rolled them back to that from 3.4.23188.
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Did you gain anything from the replies to your issue?
We haven’t heard from you for 3 days and I was wondering whether you were able to sort it based on the suggestions made 
It should be the problem of USB flash drive reading delay. I have two more questions that have not been answered for two days. I don’t know why. One is that the volume adjustment according to the vehicle speed is invalid. The other is that the Bluetooth music album icon only flashes and then disappears. The tone icon is always gray.