There are a number of reasons why a MD5 checksum in ClockworkMod Recovery might fail. Flashing different versions of the recovery can do it, flashing custom versions of the recovery can do it, or attempting to restore a Nandroid from an old phone to a replacement can do it. Sometimes, there is a need to change the MD5 checksum to make a Nandroid work again.
XDA Junior Member arhant has shared a method that makes it pretty easy to alter the checksum of a Nandroid backup so that it matches the Nandroid backup key stored in ClockworkMod Recovery for the Samsung Galaxy Fit GT5670
The method is, as mentioned, pretty simple and involves using a couple of programs to change the checksum so that it works in recovery. Users simply need a program called MD5 calculator and Notepad++. Some users may be tempted to use regular Notepad in Windows, but Notepad++ doesn’t add any codecs to the text which will prevent any compatibility problems. Once completed, the Nandroid will no longer fail checksum errors and flash as normal.
It should be noted that a bad checksum can be caused by a corrupt backup. Flashing corrupt backups can cause some serious damage to the device, including bricking it. With this in mind, exercise the utmost caution when bypassing MD5 protection. If this looks like the fix you’ve been looking for, you can find the full method and discussion in the original thread._________