Is this what you are "NON roooted"? If so, that may be your problem.
The sbf flashes fine. I'm not sure exactly HOW I finally got it to take the backup. But after pouring over various forums and seeing similar problems- the method that worked in order was to follow the typical non-root hack guide. Install the SBF and build the /nandroid/ folder with whatever version of the OS you intend to replace, yada yada... just as the guides suggest.
BUT, when the phone initially flashes after the SBF is loaded with RSD Lite, and the phone goes to reboot, IMMEDIATELY hold the "x" key down to enter the nandroid loader. To not let it reboot on its own and go through the power cycle routine again. That seemed to keep the SD card mounted. I tried all other 'remove the battery' 'clear the cache/data', reseat the SD card, etc. etc. multiple times. At any rate- I hope this helps- The "existing with 20 error" is a simple matter of not having the SD card mounted... and although a day (year or two) late and a few models short- In Mexico, the A854 droid (called Motoroi here.- but identical to the Verizon version.) is still sold through corporate plans... and it NEVER receives OS updates from the carrier Iusacell - so brute force is the only method. Planning on relocating back to the U.S. soon and will definitely buy something more 'this century.' At any rate the guide listed at A Guide to get Froyo - for NON-ROOTED users - MyDroidWorld
works, it's just a really stubborn phone sometimes. Phone is now at 2.2 and rooted via this method. The SBF hack file used at the above link "SD mount" option doesn't seem to 'hold' once you leave that particular menu.... and return to the backup/restore sections... that was the 'stuck' point for me. My phone thinks it's from Verizon now... but functions 100% as an Iusacell phone... plus now I get the cool 'droid' voices and sounds instead of the local carrier's rather annoying jingle.