There are several easy things I would suggest.
Maybe you used Jfilerecovery for this already, but there are definitely other programs that will copy & save the file up to the point with the error, where it will then probably stall, but you'll have a good portion of the file sitting on your hard drive without it disappearing like it would from a Windows file-copy. You can then just cancel or exit out of the transfer once it stalls and the copied portion will stay in tact.
Then you at least have a lossless digital copy of the first half of the file, which is one step that's covered if you can't retrieve the rest of it.
The next thing I would do is try playing the file on the iRiver itself first, and see what happens when it gets to that point. Then I would also see if the iRiver allows skipping around the file, and maybe you could skip past the error and be able to play the file from after the error point until the end. Then you could at least get an analog copy of most of the 2nd half of the show.
If that didn't work, I would hook up the iRiver to the PC via USB etc, and try playing the file within Windows via your usual software... Media Player Classic, VLC, Winamp etc, and see what happens. It will probably stop or error-out when it gets to the problem spot, but those programs can usually let you skip past the error if you're careful enough to skip completely past it, and then you'll still be able to play the rest of the show. Then you could do a real-time recording within Windows of the remaining set and you might only lose 1-2 songs (or less) due to the error.
Then I would attempt other options of fixing the overall problems. My first priority would be to secure as much of the recording as possible first and/or make sufficient copies before you attempt something that could possibly change/damage that area of the HDD further. That's what I would do first, no question.
Lastly, when that's done you may also want to bring your iRiver to a friend's house with a Mac or Linux system. Someone talked about Linux already, but even Mac's seem to work differently when copying corrupted files. They won't error-out, it will just copy the file even if some portions are damaged. If the entire file was damaged (which we know it's not), then the copied file might just be a garbled mess, but in the case of one error spot, the beginning should be completely fine and maybe all the portions after the error would be fine too. Do that and then skip around the file to see if there's just a slight skip during the part with the error. One problem is you don't know yet how big the error portion is, but all of these things will help answer that.
I've had experience doing all of these things to various extents to help recover files, so I hope it helps.