I tried to restore a file I earlier created using "d2s" and "ntrw read", and couldn't get the "Press power to flash" prompt to appear. After three hours of work I know now why!
Note: I have a rather old Magician which still uses the Bootloader version 1.00. HTC may have fixed what I'll describe below for 1.02, so maybe it doesn't apply to most of you.
The problem turned out to be that when you call "ntrw write ...", it does not start writing at the very beginning of the SD card, but instead at the beginning of the first partition on that card, leaving the Master Boot Record (MBR) intact!
It seems that the boot loader (at least v 1.00) is unable to skip over the MBR - it tries to read the Flash image directly from the beginning of the SD card, does not find the signature 'MAGICIAN' there, decides it's not an upgrade SD card, and does not display the "Press power to flash" prompt.
Now, how can this be solved? None of the formatting tools I tried would touch (i.e., delete) the MBR on the SD card, not even the one in my digital camera or the Magician formatter itself. But fortunately there is an easy solution: use romupdate.exe and the d2s command to dump your Magician's ROM to the SD card. This indeed does destroy the MBR, and then ntrw will start writing right from the very beginning of the SD card, as verified with WinHex.
After that, flashing my old backed up ROM went flawlessly.
Bottom line: before using "ntrw write ...", prepare your SD card with romupdate.exe and d2s - and do not format it until you are finished with the whole process!
Oh, and don't worry: Windows will recreate the MBR when you format you SD card later.