注意，U盘用Fat32格式，NTFS格式的话，需要在Linux另外安装相应驱动。
可通过udev实现如题的功能。
在/etc/udev/rules.d/目录下新建规则文件98-logcopy.rules
内容如下：
KERNEL!="sd[a-z][0-9]", GOTO="media_by_label_auto_mount_end" # Import FS infos
IMPORT{program}="/sbin/blkid -o udev -p %N" # Get a label if present, otherwise specify one
ENV{ID_FS_LABEL}!="", ENV{dir_name}="%E{ID_FS_LABEL}"
ENV{ID_FS_LABEL}=="", ENV{dir_name}="usbhd-%k" # Global mount options
ACTION=="add", ENV{mount_options}="relatime" # Filesystem-specific mount options
ACTION=="add", ENV{ID_FS_TYPE}=="vfat|ntfs", ENV{mount_options}="$env{mount_options},utf8,gid=100,umask=002" # Mount the device
ACTION=="add", RUN+="/bin/mkdir -p /media/%E{dir_name}", RUN+="/bin/mount -o $env{mount_options} /dev/%k /media/%E{dir_name}" RUN+="/bin/cp -r /opt/trash/logs /media/%E{dir_name}" # Clean up after removal
ACTION=="remove", ENV{dir_name}!="", RUN+="/bin/umount -l /media/%E{dir_name}", RUN+="/bin/rmdir /media/%E{dir_name}" # Exit
LABEL="media_by_label_auto_mount_end"
可根据实际情况，修改挂载后的处理脚本。
保存文件并重启，插入U盘测试，发现并未成功的拷贝日志。
通过如下命令查看日志
systemctl status udev
看到有如下错误提示：
pi@raspberrypi:~ $ systemctl status udev
● systemd-udevd.service - udev Kernel Device Manager
Loaded: loaded (/lib/systemd/system/systemd-udevd.service; static; vendor preset: enabled)
Active: active (running) since Sat 2019-08-10 16:31:27 CST; 6h ago
Docs: man:systemd-udevd.service(8)
man:udev(7)
Main PID: 128 (systemd-udevd)
Status: "Processing with 16 children at max"
CGroup: /system.slice/systemd-udevd.service
└─128 /lib/systemd/systemd-udevd Aug 10 21:27:15 raspberrypi mtp-probe[8780]: checking bus 1, device 8: "/sys/devices/platform/soc/3f980000.usb/usb1/1-1/1-1.1/1-1.1.2"
Aug 10 21:27:17 raspberrypi systemd-udevd[8785]: Process '/root/mount_manager/mount_manager add' failed with exit code 1.
Aug 10 21:27:30 raspberrypi mtp-probe[8822]: checking bus 1, device 9: "/sys/devices/platform/soc/3f980000.usb/usb1/1-1/1-1.3"
Aug 10 21:27:49 raspberrypi systemd-udevd[8871]: Process '/root/mount_manager/mount_manager remove' failed with exit code 1.
Aug 10 22:34:07 raspberrypi systemd-udevd[10802]: Process '/bin/umount -l /media/usbhd-sda1' failed with exit code 32.
Aug 10 22:34:07 raspberrypi systemd-udevd[10802]: Process '/bin/rmdir /media/usbhd-sda1' failed with exit code 1.
Aug 10 22:34:43 raspberrypi mtp-probe[10815]: checking bus 1, device 10: "/sys/devices/platform/soc/3f980000.usb/usb1/1-1/1-1.1/1-1.1.2"
Aug 10 22:34:45 raspberrypi systemd-udevd[10819]: Process '/bin/mkdir -p /media/usbhd-sda1' failed with exit code 1.
Aug 10 22:34:45 raspberrypi systemd-udevd[10819]: Process '/bin/mount -o relatime,utf8,gid=100,umask=002 /dev/sda1 /media/usbhd-sda1' failed with exit code 32.
Aug 10 22:34:45 raspberrypi systemd-udevd[10819]: Process '/bin/cp /opt/trash/logs /media/usbhd-sda1' failed with exit code 1.
经过Google查询，原来树莓派的系统有点Bug。关于此问题的详细信息 看这里
解决方法：
sudo nano /lib/systemd/system/systemd-udevd.service
打开上面的文件，将后7行注释掉，就可以了。否则udev规则虽会触发执行，但会执行失败。
修改后的文件内容如下：
# This file is part of systemd.
#
# systemd is free software; you can redistribute it and/or modify it
# under the terms of the GNU Lesser General Public License as published by
# the Free Software Foundation; either version 2.1 of the License, or
# (at your option) any later version. [Unit]
Description=udev Kernel Device Manager
Documentation=man:systemd-udevd.service(8) man:udev(7)
DefaultDependencies=no
Wants=systemd-udevd-control.socket systemd-udevd-kernel.socket
After=systemd-udevd-control.socket systemd-udevd-kernel.socket systemd-sysusers.service
Before=sysinit.target
ConditionPathIsReadWrite=/sys [Service]
Type=notify
OOMScoreAdjust=-1000
Sockets=systemd-udevd-control.socket systemd-udevd-kernel.socket
Restart=always
RestartSec=0
ExecStart=/lib/systemd/systemd-udevd
#KillMode=mixed
#WatchdogSec=3min
#TasksMax=infinity
#MountFlags=slave
#MemoryDenyWriteExecute=yes
#RestrictRealtime=yes
#RestrictAddressFamilies=AF_UNIX AF_NETLINK AF_INET AF_INET6
再次保存，重启。
将U盘插入树莓派，已经成功复制了日志文件。
现在也还有不完善的地方，比如，设备如果有多套，往U盘拷贝日志的路径是一样的，那么永远只能拿到最后插的那台设备的日志。所以应该能在Copy日志的时候用与设备相关的名称做为目录对日志文件进行管理。
按如上设想修改后，就可以用一个U盘从现场取N个设备的日志了。
我又想到，也可以反方向的，像安卓手机卡刷一样，用U盘升级系统。
===================================================================================================
2019-08-11更新 实现上面末尾提到的两个功能
由板载程序在/opt/trash/boxid.txt位置创建一个保存设备编号的文件。板载程序通过登录应答消息从服务端得到设备编号，并写入到上述文件。
再把规则文件修改一下，挂载完U盘后执行一个Shell脚本，修改后内容如下：
KERNEL!="sd[a-z][0-9]", GOTO="media_by_label_auto_mount_end" # Import FS infos
IMPORT{program}="/sbin/blkid -o udev -p %N" # Get a label if present, otherwise specify one
ENV{ID_FS_LABEL}!="", ENV{dir_name}="%E{ID_FS_LABEL}"
ENV{ID_FS_LABEL}=="", ENV{dir_name}="usbhd-%k" # Global mount options
ACTION=="add", ENV{mount_options}="relatime" # Filesystem-specific mount options
ACTION=="add", ENV{ID_FS_TYPE}=="vfat|ntfs", ENV{mount_options}="$env{mount_options},utf8,gid=100,umask=002" # Mount the device
ACTION=="add", RUN+="/bin/mkdir -p /media/%E{dir_name}", RUN+="/bin/mount -o $env{mount_options} /dev/%k /media/%E{dir_name}" RUN+="/opt/trash/usb %E{dir_name}" # Clean up after removal
ACTION=="remove", ENV{dir_name}!="", RUN+="/bin/umount -l /media/%E{dir_name}", RUN+="/bin/rmdir /media/%E{dir_name}" # Exit
LABEL="media_by_label_auto_mount_end"
在/opt/trash目录下创建一个shell脚本，名称为usb
shell脚本内容如下：
#!/bin/bash boxid=$(cat /opt/trash/boxid.txt)
curdt=$(date +%Y%m%d%H%M%S) sudo mkdir -p /media/$1/boxlogs/${boxid}/${curdt}
sudo cp -r /opt/trash/logs/* /media/$1/boxlogs/${boxid}/${curdt} sudo cp /media/$1/boxupdate/* /opt/trash
以上脚本先从/opt/trash/boxid.txt文件取得保存的设备编号，然后又取了当前时间，按“设备号/时间”的形式创建了目录，然后再把日志拷贝到这个目录里。
最后一句是从U盘指定目录拷贝内容到板载程序目录，实现了用U盘更新板载程序的功能。
这样，功能就完美了。
附几个命令：
udevadm monitor sudo udevadm control --reload-rules