原文地址：http://blog.pickbox.me/2013/08/28/tp-link-tl-wr703n%E8%B7%AF%E7%94%B1usb%E5%8F%A3%E7%94%A8%E9%80%942-%E6%8C%82%E8%BD%BDu%E7%9B%98%EF%BC%8C%E4%BB%8Eu%E7%9B%98%E5%90%AF%E5%8A%A8/

這裡先在自己的主Linux系統中格式化好U盤，就不用在Openwrt中格式化U盤了
掛載U盤需要安裝一些pkg模組，我的空間不夠用了，於是先恢復到刷Openwrt韌體的原始狀態（出廠設定）
重啟路由的時候按住reset，可以看到藍色LED燈閃爍，接著很快的閃爍，說明進入了恢復模式
這時用telnet登陸到192.168.1.1（電腦用網線連線到wr703n的LAN口），執行

mount_root
firstboot
reboot -f

這樣就恢復到了韌體的初始狀態

接下來安裝pkg

[email protected]:/etc/config# opkg updateDownloading http://downloads.openwrt.org/attitude_adjustment/12.09/ar71xx/generic/packages/Packages.gz.Updated list of available packages in/var/opkg-lists/attitude_adjustment.

[email protected]:/etc/config# df
 
Filesystem1K-blocks      UsedAvailableUse%Mounted on
rootfs                    108821687220%/
/dev/root                 204820480100%/rom
tmpfs                    14608       672     13936   5% /tmp
tmpfs                      51205120%/dev
/dev/mtdblock3            108821687220%/overlay
overlayfs:/overlay        1088216872
 
20%/

[email protected]:/etc/config# opkg install kmod-usb-storage block-mount kmod-fs-ext4Installing kmod-usb-storage (3.3.8-1) to root...Downloading http://downloads.openwrt.org/attitude_adjustment/12.09/ar71xx/generic/packages/kmod-usb-storage_3.3.8-1_ar71xx.ipk.Installing kmod-scsi-core (3.3.8-1) to root...Downloading http://downloads.openwrt.org/attitude_adjustment/12.09/ar71xx/generic/packages/kmod-scsi-core_3.3.8-1_ar71xx.ipk.Installing block-mount (0.2.0-9) to root...Downloading http://downloads.openwrt.org/attitude_adjustment/12.09/ar71xx/generic/packages/block-mount_0.2.0-9_ar71xx.ipk.Installing blkid (2.21.2-1) to root...Downloading http://downloads.openwrt.org/attitude_adjustment/12.09/ar71xx/generic/packages/blkid_2.21.2-1_ar71xx.ipk.Installing libblkid (2.21.2-1) to root...Downloading http://downloads.openwrt.org/attitude_adjustment/12.09/ar71xx/generic/packages/libblkid_2.21.2-1_ar71xx.ipk.Installing libuuid (2.21.2-1) to root...Downloading http://downloads.openwrt.org/attitude_adjustment/12.09/ar71xx/generic/packages/libuuid_2.21.2-1_ar71xx.ipk.Installing swap-utils (2.21.2-1) to root...Downloading http://downloads.openwrt.org/attitude_adjustment/12.09/ar71xx/generic/packages/swap-utils_2.21.2-1_ar71xx.ipk.Installing kmod-fs-ext4 (3.3.8-1) to root...Downloading http://downloads.openwrt.org/attitude_adjustment/12.09/ar71xx/generic/packages/kmod-fs-ext4_3.3.8-1_ar71xx.ipk.Installing kmod-lib-crc16 (3.3.8-1) to root...Downloading http://downloads.openwrt.org/attitude_adjustment/12.09/ar71xx/generic/packages/kmod-lib-crc16_3.3.8-1_ar71xx.ipk.Configuring kmod-scsi-core.Configuring kmod-usb-storage.Configuring kmod-lib-crc16.Configuring libuuid.Configuring libblkid.Configuring blkid.Configuring swap-utils.Configuring block-mount.Configuring kmod-fs-ext4.

[email protected]:/etc/config# df -hFilesystemSizeUsedAvailableUse%Mounted on
rootfs                    1.1M640.0K448.0K59%/
/dev/root                 2.0M2.0M0100%/rom
tmpfs                    14.3M    680.0K     13.6M   5% /tmp
tmpfs                   512.0K0512.0K0%/dev
/dev/mtdblock3            1.1M640.0K448.0K59%/overlay
overlayfs:/overlay        1.1M640.0K448.0K59%/

這裡可以看到安裝pkg用了大概400多K的空間

[email protected]:/etc/config# ls /dev/
bsg        mtd0ro     mtd4ro     mtdblock5  sda1       ttyS12     ttyS6
bus        mtd1       mtd5       null       shm        ttyS13     ttyS7
console    mtd1ro     mtd5ro     port       tty        ttyS14     ttyS8
full       mtd2       mtdblock0  ppp        ttyATH0    ttyS15     ttyS9
kmsg       mtd2ro     mtdblock1  ptmx       ttyS0      ttyS2      urandom
log        mtd3       mtdblock2  pts        ttyS1      ttyS3      watchdog
mem        mtd3ro     mtdblock3  random     ttyS10     ttyS4      zero
mtd0       mtd4       mtdblock4  sda        ttyS11     ttyS5

可以看到在/dev目錄下多了sda和sda1，說明U盤被識別出來了
修改/etc/config/fstab檔案：

[email protected]:/etc/config# vi fstab
config global automount
        option from_fstab 1
        option anon_mount 1

config global autoswap
        option from_fstab 1
        option anon_swap 0

config mount
        option target   /overlay
        option device   /dev/sda1
        option fstype   ext4
        option options  rw,sync
        option enabled  1
        option enabled_fsck 0

config swap
        option device   /dev/sda2
        option enabled  0

執行下面命令使其生效：

[email protected]:~# /etc/init.d/fstab enable

接下來將系統root內容拷貝至U盤：

mount /dev/sda1 /mnt
mkdir /tmp/root
mount -o bind / /tmp/root
cp -a /tmp/root/* /mnt/
umount /tmp/root

修改U盤上的/mnt/etc/banner，最末尾處加一行boot from usb，標識是從U盤啟動的系統

[email protected]:~# vi /mnt/etc/banner
  _______                     ________        __
 ||.-----.-----.-----.||||.----.||_
 |-||  _  |-__|||||||   _||   _||_______||   __|_____|__|__||________||__||____||__| W I R E L E S S   F R E E D O M
 -----------------------------------------------------
 ATTITUDE ADJUSTMENT (12.09, r36088)-----------------------------------------------------*1/4 oz VodkaPour all ingredients into mixing
  *1/4 oz Gin        tin with ice, strain into glass.*1/4 oz Amaretto*1/4 oz Triple sec
  *1/4 oz Peach schnapps
  *1/4 oz Sour mix
  *1 splash Cranberry juice
 -----------------------------------------------------
 boot from usb
 -----------------------------------------------------

重啟路由後就可以使用了

[email protected]:~# reboot
[email protected]:~# df -hFilesystemSizeUsedAvailableUse%Mounted on
rootfs                    7.4G248.7M6.8G3%/
/dev/root                 2.0M2.0M0100%/rom
tmpfs                    14.3M    380.0K     13.9M   3% /tmp
tmpfs                   512.0K0512.0K0%/dev
/dev/sda1                 7.4G248.7M6.8G3%/overlay
overlayfs:/overlay        7.4G248.7M6.8G3%/

可以看到，系統的儲存空間變為7.4G了！

注：重刷openwrt韌體後還想使用原來U盤，可能會碰到這樣一個錯誤：/dev/sda1被掛載到了/tmp/overlay-disabled，解決辦法是

cp /.extroot.md5sum /tmp/overlay-disabled/etc/extroot.md5sum