前言

在平時使用openwrt的過程中，往往會出現flash不夠用的情況，畢竟一般的路由器只有16M的記憶體。這個時候就需要外部儲存器。我這裡選擇U盤。在掛載U盤的時候出現一些失誤，現已文章記錄，以便各網友借鑑，避免走彎路。

步驟：
1.格式化U盤為ext4格式，也可以為ext3。其他的格式未作實驗。

命令：fdisk /dev/sda1 

[email protected]:/# fdisk /dev/sda1

Welcome to fdisk (util-linux 2.24.1).
Changes will remain in memory only, until
 
 you decide to write them.
Be careful before using the write command.

Device does not contain a recognized partition table.

Created a new DOS disklabel with disk identifier 0x2ba49c96.

Command (m for help): m

Help:

  DOS (MBR)
   a   toggle a bootable flag
   b   edit nested BSD disklabel
   c   toggle the
 
 dos compatibility flag

  Generic
   d   delete a partition
   l   list known partition types
   n   add a new partition
   p   print the partition table
   t   change a partition type
   v   verify the partition table

  Misc
   m   print this menu
   u   change display/entry units
   x   extra functionality (experts only)

  Save & Exit
   w   write
 
 table to disk and exit
   q   quit without saving changes

  Create a new label
   g   create a new empty GPT partition table
   G   create a new empty SGI (IRIX) partition table
   o   create a new empty DOS partition table
   s   create a new empty Sun partition table

選擇新建一個分割槽： n

Command (m for help): n

Partition type:
   p   primary (0 primary, 0 extended, 4 free)
   e   extended
Select (default p): p

儲存分割槽表： w

Command (m for help): w

2，分割槽完成後，用格式化，

root@OpenWrt:/# mkfs.ext4 /dev/sda1

3，嘗試掛載

root@OpenWrt:/# mount -t ext4 /dev/sda1 /mnt
root@OpenWrt:/# 
root@OpenWrt:/# umount

若沒有錯誤則說明分割槽成功，可進行下一步。

4，修改/etc/config/fstab

[email protected]:/# vim /etc/config/fstab 

config global
        option anon_swap '0'
        option anon_mount '1'
        option auto_swap '1'
        option auto_mount '1'
        option delay_root '5'
        option check_fs '0'

config mount
        option uuid 'e349995e-911d-47e8-90b1-45412826c62a'
        option enabled '1'
        option target '/'
        option device '/dev/sda1'
        option fstype 'ext4'
        option is_rootfs '1'
        option enabled_fsck '1'

值得注意的地方：
a：option uuid 這個選項是為了避免同時掛接幾個硬碟時出現錯誤。
檢視命令 blkid 磁碟型別UUID

root@OpenWrt:/# blkid
/dev/mtdblock4: TYPE="squashfs" 
/dev/sda1: UUID="e349995e-911d-47e8-90b1-45412826c62a" TYPE="ext4" PARTUUID="6e697373-01" 
root@OpenWrt:/# 

5，複製所有的檔案到U盤裡：

root@OpenWrt:~# mkdir /tmp/root
root@OpenWrt:~# mount -o bind  / /tmp/root
root@OpenWrt:~# cp  /tmp/root/* /mnt/sda1 -a
root@OpenWrt:~# umount /tmp/root/

6，開機重啟完成，最後驗證了

BusyBox v1.22.1 (2015-04-11 23:12:20 CST) built-in shell (ash)
Enter 'help' for a list of built-in commands.

  _______                     ________        __
 |       |.-----.-----.-----.|  |  |  |.----.|  |_
 |   -   ||  _  |  -__|     ||  |  |  ||   _||   _|
 |_______||   __|_____|__|__||________||__|  |____|
          |__| W I R E L E S S   F R E E D O M
 -----------------------------------------------------
 BARRIER BREAKER (Barrier Breaker, r44952)
 -----------------------------------------------------
  * 1/2 oz Galliano         Pour all ingredients into
  * 4 oz cold Coffee        an irish coffee mug filled
  * 1 1/2 oz Dark Rum       with crushed ice. Stir.
  * 2 tsp. Creme de Cacao
 -----------------------------------------------------
 ----BOOT--FROM--USB--DEVRICE------------Lijg---------
 -----------------------------------------------------

[email protected]:~# 
[email protected]:/# df -h
Filesystem                Size      Used Available Use% Mounted on
rootfs                   14.3G     89.0M     13.4G   1% /
/dev/root                13.3M     13.3M         0 100% /rom
tmpfs                    30.3M    804.0K     29.5M   3% /tmp
/dev/sda1                14.3G     89.0M     13.4G   1% /
tmpfs                   512.0K         0    512.0K   0% /dev
/dev/mtdblock5            1.4M    452.0K    956.0K  32% /mnt/mtdblock5
[email protected]:/# mount
rootfs on / type rootfs (rw)
/dev/root on /rom type squashfs (ro,noatime)
proc on /proc type proc (rw,noatime)
sysfs on /sys type sysfs (rw,noatime)
tmpfs on /tmp type tmpfs (rw,nosuid,nodev,noatime)
/dev/sda1 on / type ext4 (rw,relatime,data=ordered)
tmpfs on /dev type tmpfs (rw,relatime,size=512k,mode=755)
devpts on /dev/pts type devpts (rw,relatime,mode=600)
debugfs on /sys/kernel/debug type debugfs (rw,noatime)
/dev/mtdblock5 on /mnt/mtdblock5 type jffs2 (rw,relatime)
[email protected]:/# 

最後可看到16G的U盤掛載啟動成功了。