[Touch Diamond, MDA Compact IV ROM Development] danifilth4king:
Hope I can help!

Hey! First to 'avior.almo' You need to make sure you have flashed HardSPL to your device before trying to flash ANY ROM to your device (cooked, or custom, or otherwise!) If you are unsure of what I mean, or have any trouble, see sticky threads #2 and #4 of 'Diamond ROM development' http://xdaforums.com/forumdisplay.php?f=430 (I use HardSPL Olinex 1.40)

Once you have this all in place, then flashing from the SD card is easy; you simply need to download the ROM you wish to flash, extract it (if it's downloaded as a .zip or .rar file) so you can find an '.nbh' file. The '.nbh' file is the ROM itself, and the file that you wish to flash. Once you have extracted the ROM you wish to flash (if you needed to extract at all. You may have downloaded the .nbh file as it is) then whatever the .nbh file name is, you need to rename the .nbh file to 'DIAMIMG.nbh' (without quotation marks).

Once you have done that, you need to copy it to your device's internal memory. To do this, connect your Diamond via USB, and when asked on your device screen, choose the 'Disk Drive' type of connection. Once complete, you can then navigate to the root of your (SD card) Internal Storage by accessing 'My Computer' through Windows explorer, Your device drive letter may be different depending on how many hard disks/CD drives you have in your system. (Mine is drive E:) Anyway, access your internal storage drive, whatever letter it may be on your system. It will appear under 'Drives with removable storage'. This is where you need to copy the 'DIAMIMG.nbh' file to. (To rdelsol: No, you don't need to clear your internal storage of any files or folders, as long as you have enough space in your internal storage to paste the ROM (.nbh file) that you wish to flash, in the root directory. E.g. 'E:' drive. It can be in there along with your 'program files' folder, or your 'my pictures' or 'my videos' folders. Flashing shouldn't delete any files or folders you have in your internal storage either. Even after flashing, 'DIAMIMG.nbh' should also still be there! (After flashing, you can delete that file)) And I noticed that in your post, you typed 'DIAMING.nbh' whether that was just a typo or not, I don't know, but it needs to be 'DIAMIMG.nbh', if so, that may be where you're going wrong.

So, now that we have the ROM image file in the root directory of your internal storage, you need to enter your device into bootloader mode, which is achieved by (while the device is on) holding down the 'VolDown' and 'back' (<-) buttons, while pressing the little red reset button (under the battery cover). Keep hold of the voldown and back buttons until your device displays the tri-colour screen. The instructions are then shown on-screen of what to do next (PROVIDING YOU HAVE HARDSPL INSTALLED, see beginning of post). Your device should detect the ROM image file (saved as DIAMIMG.nbh) and prompt you to press the power button to begin the flash. All going well, your device will display a progress bar, that when reaches 100% will say 'Update Success'

Now you need to press the red reset button again. After that, your device should begin setup of the new ROM as if it had just been bought. Alignment of screen, and maybe time and date setup, for example. Good luck with flashing and setup!