Hey @r_knipp had olat of issues with this as well perhaps not a usb but you need to separate how squeezelite start and what it uses
Here is my working squeezelite systemd service file hope this helps it lets you play music on squeezelite when system is idle you can change it to type simple if its not working for you.
Also you might wanna change it audiocard by changing sysdefault:CARD=ALSA to desired audiocard by checking what interfaces are available with squeezelite -l
place this file at /etc/systemd/system/squeezelite.service and change squeezeboxserver to ip or dns for your server and clientname to whatever you want your client to be named
[Unit]
Description=SqueezeLite Player Service
After=network.target
[Service]
Type=idle
User=root
PAMName=system-local-login
ExecStart=/usr/bin/squeezelite -o "sysdefault:CARD=ALSA" -s squeezeboxserver:3483 -n "clientname"
ExecStop=/bin/kill -SIGTERM $MAINPID
[Install]
WantedBy=multi-user.target
then
cd /etc/systemd/system/
sudo systemctl enable squeezelite.service
sudo service squeezelite start
also clear out the old init.d script
sudo /etc/init.d/squeezelite stop
sudo update-rc.d squeezelite disable
sudo rm /etc/init.d/squeezelite