Jocky's HardSPL page is
Here. I'd suggest using the 1.24 version.
Make sure the Diamond is connected via ActiveSync/WMDC (NOT as a Disk Drive) and run the included Batch file followed by the RUU Updater for the HArdSPL you want to install.
From there, retry the Radio update. If that wont work, it may be worth trying to flash it from the Diamond itself, rather than via the PC. To do this you need to get the correct file onto the Diamond; locate the radio****.nbh file from the download and rename it to DIAMIMG.nbh (probably safer to create a copy first!); use ActiveSync or Disc Drive mode to copy this file to the root of the Internal Storage.
Now, disconnect from the PC and get the Diamond into Bootloader mode. The BL will see the new DIAMIMG.nbh file and ask if you want to update. Press the required button, and it will proceed.
You can also follow the same route with a full ROM update- any DIAMIMG.nbh file in the root of Internal Storage will be seen by the Bootloader, so as before, rename the ROM update *.nbh file to DIAMIMG.nbh and put it in Internal Storage.
Remember to move the 'used' DIAMIMG.nbh file somewhere safe to avoid unwanted activity! I keep my nbh files in a folder on the Diamond's Internal Storage in case I need a quick ROM flash away from the PC
