Unable to update software in DSM300

I have a networked system including 2x E120 classics (SW vers 5.69) and a DSM300 with SW version 3.16. Trying to upgrade the DSM300 to latest version 4.20. I have downloaded that package plus the 5.18 boot loader onto the memory card. Software utility window opens on power up. Attempts to install the boot loader fail. The progress window opens, however no text or progress bar data is updated. Approximately 30 seconds later an additional window with "the data transfer to the target has failed"

Tried multiple times with same result.
Any help or ideas would be appreciated.

Thanks
Mike