Thanks to Sukuwc for posting the zipfile for app note AVR42788 !! As a result, I was able to get a XMEGA bootloader running from SD card on my telescope controller project.
However, I discovered that one of three of my prototype controllers is temperamental. If I leave it off for an hour, and plug it in and bootload, it will work perfect every time. Right then it can also be programmed again and again for a little while -- but once it "warms up" though, it starts getting FAT read errors, eventually to the point it will not be able to succeed ever. Until I unplug it and walk away for a while.
My other 2 controllers work perfectly every time.
So the only components involved are the uC and the SD card, some pullup resistors, and some very small series resistors. I reflowed every joint between them, but it didn't change anything.
I swapped SD cards around and even brands and sizes, but it didn't change anything.
Can this really be the uC acting bizarrely? I am kinda loathe to go and replace the 64 pin uC.
How much capacitance should I have on the SD power? I have a 10uF and a 0.1 uF. I am guessing that should be more like 47uF or more?
Thanks for your help
Mike in Alaska