Linux下安全移除USB存储设备

If you are using systemd then use udisksctl utility with power-off option:

power-off

Arranges for the drive to be safely removed and powered off. On the OS side this includes ensuring that no process is using the drive, then requesting that in-flight buffers and caches are committed to stable storage.

I would recommend first to unmount all filesystems on that usb. This can be done also with udisksctl, so steps would be:

udisksctl unmount -b /dev/sda1
udisksctl power-off -b /dev/sda