Ok, I've just ran into the same problem an for over an hour, nobody was really happy to help me, so I'll try to be constructive for everyone who would be in the same problem as me.
My situation:
I've a Sansa e280 American player (with the radio), I've installed RockBox but I was unable to remove it.
I just did the dan_a procedure, but I've lost the RADIO as I installed an European firmware on an American player.
Note: It would NOT be a good idea to install an American firmware on an European player. Based on what I've read, the European player doesn't have the "radio chip"...
So, if you need the original American Firmware, use this one:
http://daniel.haxx.se/sansa/e200/1.02.15a.mi4... and follow the dan_a procedure
Note #2: if you copy a bad file in the 16mb partition, you will broke the player. Only copy VALID .mi4 files.
Here is a recap of the dan_a procedure:
1) Put it into recovery mode by switching it on with "hold" on while pressing the record button.
Plug it into the USB on your computer.
2) Open "My Computer." There should be a 16MB drive. There won't be many files on there, but don't worry, all your files are still in the Sansa.
3) Copy ONLY original .mi4 file onto the 16MB drive. Do not copy anything else. Really. I mean it.
Original .mi4 files:
European:
http://daniel.haxx.se/sansa/e200/SKU_E-PP5022.mi4 American:
http://daniel.haxx.se/sansa/e200/1.02.15a.mi4 4) Safely disconnect the Sansa from the USB.
5) Send a PayPal donation to the Rockbox project.