1、出现故障的原因：
A、通过手动更换镜像导致
Boot Loader Menu
Run primary image (7.0.220.0) - Active
Run backup image (7.0.220.0)
Change active boot image
Clear configuration
Format FLASH Drive
Manually update images
Enter selection: 6
B、版本之间不能直接升级导致
C、Flash被erase了
故障现象：
.......
loading primary image(Bad image type)
Launching...
Launching images...
2、解决方法：
开机ESC，选6：Manually update images；
注意：该模式下无法解密.aes文件。需要单独加载RTOS进行升级然后进入系统升级。
所需文件，如下示例（可能版本有不同）：
enter filename for RTOS update: AS5500RTOS-7.0.250.0RTOS
Enter filename for FP update: AS5500caviummain-7.0.250.0
获取方式：通过思科官方获取（联系思科TAC支持，获取）。
另：该云盘下有7.0.250.0的RTOS
http://pan.baidu.com/s/1ntqPeh7
3、安装细节：
Boot Loader Menu
============================================================
Run primary image (Image not found) - Active
Run backup image (Image not found) This shows that there are no valid images in either Primary or Secondary partition, and that the system is configured to boot from Primary.
Change active boot image
Clear configuration
Format FLASH Drive
Manually update images
Enter selection: 6
Launching...
Launching images...
init started: BusyBox v1.6.0 (2009-02-03 04:56:32 EST) multi-call binary
Mount failed for selinuxfs on /selinux: No such file or directory
starting pid 655, tty '': '/etc/init.d/rcS'
Use DHCP for ip configuration (Y/n)? n
Enter switch IP address: 192.168.1.100
By "switch", the boot loader means WLC
Enter switch netmask: 255.255.255.0
Enter switch gateway: 192.168.1.1
Enter TFTP server IP address: 192.168.5.157
You have entered:
IP Address : 192.168.1.100
Netmask : 255.255.255.0
Gateway : 192.168.1.1
Server Ip Addr : 192.168.5.157
Is this correct(y/N)? y
!!! WARNING updating using .aes or unapproved files will disable this unit !!!
Do you want to update RTOS (y/N)? y
Do you want to update Primary Or Secondary Image (P/s)? P
Enter filename for RTOS update: AS_5500_RTOS-7.0.250.0RTOS
update Done
Enter filename for FP update: AS_5500_cavium_main-7.0.250.0
Depending on your WLC model / boot loader version, you may not be prompted for FP update
FP update Done
Do you want to update an AP image (y/N)? N
AP Images Not UpdatedDone.
Restarting...
Restarting system.
WLCNG Boot Loader Version 1.0.15 (Built on Nov 23 2010 at 07:51:36 by cisco)
Board Revision 0.0 (SN: PSJ1438XXXX, Type: AIR-CT2504-K9)
(W)Verifying boot loader integrity...
OK.
然后再按照正常路径去升级吧。