To play music on USB drive , you need set your music library first.
Side Menu > Settings > Music Library
Add Music Library
Setup New Library
Please choose the server you want to setup.
Here we are using ARIES in my office , so I will choose "Yes, Let's setup lightning Server"
Select Lightning Server which your will insert a usb drive , tap Next
Select USB Drive, tap Next （if you want to add a network folder you could choose another one ）
Insert your USB storage device, Next
System will scan the music for awhile.(Depends on the number of files in your storage devices)
To check Music Library connected by USB Drive or Network Folder, please go
Side Menu > Settings > Music Library > [USB Drive] / [Your Lightning Server]
It also will display the storage states.