I ran into a little snag installing Rockbox, and read here that others have recently had this problem, too. Since I can't edit the manual, I thought I'd post my findings here for posterity and for the manual writers.
In order to get Rockbox working, I had to
reflash my H320
after unzipping Rockbox to the player's root directory. To be clear here's what I did:
1. downloaded compatable firmware (I chose 1.29J) from iRiver's website
2. download the Rockbox firmware patcher from rockbox.org
3. patch the default firmware as described in the Rockbox manual
4. load the firmware into the player's root directory (NB: manual didn't mention this, but there was already a file named H300.hex in that directory; I renamed it H300.hex.bak and put the modified H300.hex file in the root directory; the player started up without any problems)
5. upgrade firmware as described in the Rockbox manual
6. plug player in, unzip latest Rockbox build and fonts package into root directory
Now, at this point, I at first had trouble turning the player back on. I fiddled with it a while, and it finally came on when I plugged the adapter in. After that I had no trouble turning in on (even without the adapter), but it always booted into the iRiver 1.29J firmware. I had to:
7. without recopying any files, choose Firmware Update from iRiver's menu
The firmware updated a second time, and the player shut off when done (as it should). Next time I turned the player on, it booted right into Rockbox.
Looking forward to messing with Rockbox!
