The checksum error is simply because you have the wrong filesizes specified. You're making people cut off the end of the file. This is how I totally destroyed my SMT5600 firmware. Now that I've been experimenting a bit, I've finally had success in backing up/restoring a rom. The way I did it was using an external card reader and WinHex.
First step is to put your card into your reader and fill the entire card with 0's.
Second step is to boot up in 3-color mode and then using Tera Term issue the "r2sd all" command. It'll show you a bunch of info (including the correct filesizes in hex value) as it runs the backup.
Third step is to shutdown the phone and remove the card. Then put the card into your reader and find the boundary where the data stops and all the 0's are left from before. Select all the data up to this point and tell WinHex to copy it to a new file. You can then save this file as your backup.
To restore a phone, you just write out a new card with all 0's like we did before, then use the Disk Clone feature in WinHex to copy the data from the file onto your SD card. Leave all the offsets set to 0.
When you put this card in a phone and boot up into 3-color mode, it'll automatically prompt you to restore from the card by pressing vol-down. Press it and you should not see any checksum error, and you'll see a series of status bars during the restore.
Remember that in WinHex whenever you are working with your SD card, to open the Physical drive, NOT the logical drive.
I have done this process a few times with no problem at all, the key is to just make sure you don't cut off any data, that will obviously change the checksum value and give you an error.
Now if anyone could do this for an SMT5600 I would be very greatful.
