Yeah! I actually did it!
Thanks to dobbley's clarification I felt sure enough to pull it off. When you look at the pictures and the crappy setup you'll understand, that I propably would not have tried it if I hadn't a backup H140 in my closet... :-)
The "cable" you see on the picture isn't actually a cable but that thing that is often used to hold together rolled up cables when you buy them. I chose that thing because the metal inside is quite rigid, so I would feel resistance when I bend it at one end by almost 90° and use that hook to get hold of "the other side" of R18, the side that is faced away and dobbley said would be the side away from VCC.
It took me some time and many pictures, that I zoomed in to get a better mental picture how the feeling in my hand of the resistance when pulling the hook against the back of (hopefully) R18 would match the actual situation. I have to admit, that I probably never really saw the hook as clearly as I would have wanted, but in the end I feld secure enough to connect the player to the power, switch it on and write on the EEPROM.
The other end of the "cable" touches (as proposed by dobbley) the metal part of one of the screw holes for grounding.
As for the sequence of the setup:
I removed the accu and the hdd I normally use, in oder to not damage them in case something went wrong. Looking at the setup, you'll understand why :-). I tried without hdd, but apparently that doesn't work. So I used the old, original hdd that has still an old version of Rockbox on it.
I connected the power cord (but did not yet put the plug into the socket!) and the usb connector to the H140 in order to not to have to move the player around (involuntarily) once the "cable" was in place. Also in the final setup the player was kind of stuck under the window so it wouldn't move when I hit the "on" (play) button. I also connected the other usb connector to the pc.
Once this setup was in place and I felt comfortable with what I saw and felt in my finger tips, I plugged in the power connector, switched on the player and executed the isd300_udma.exe. No sparks, no smoke, no fire. I could enter the EEPROM writing mode and change the setting to 4. All went fine, and now I can access / load my H140 via USB about 60% faster.
Thanks to all that made this hack possible! And a special thanks again to dobbley for giving me the final confidence I needed to pull that off!
Sorry for the setup picture. I had to delete some picture information in order to get below the allowed file size without reducing any more of the picture quality or excerpt in order for it to be as "understandable" as possible. And don't mind the file names. They only really make sense to me. :-)
Please consult the picture from this post (
http://forums.rockbox.org/index.php/topic,20402.msg186903.html#msg186903) for further and more detailed visual information.
[Edit: December 6th, 2014]
Today, almost 2 years after "hacking" my first (and still running) H140, I "hacked" my spare (and still unused) H140 using my own "manual" (above). It worked again. Only this time I used a vice with rubber-protected jaws to fixate the player which made the job so much easier and faster. I didn't see any more than last time though... :-)
Btw., as I didn't mention it before and it is a crucial part of the operation: I didn't "switch off" the player after the successful operation, but I removed the hdd/player from the windows system using the USB-icon and then just pulled the power connector from the wall socket. It's brute force, I know. The HDD should (and did) survive this one time stunt. Of course, you better use a spare hdd (the one that came with the player) instead of your big, expensive hdd you just bought.
I still wonder why iRiver didn't activate UDMA in the first place, or at least made the write-protection more user-friendly.
Too bad it's "only" UDMA4, not 6....
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