"SD Write Error - Please Power Off" -- solved (for me).
I was having this problem when I tried perform a "Mixdown" of all eight tracks. It would start mixing up the music, playing things out of order for a few seconds, and then crash and give me the "SD Write Error - Please PWR OFF".
For some odd reason, it was solved when I did the following:
1. From the MULTI TRACK screen, go to MENU (press the MENU button)
2. Go to SONG
3. Go to COPY
4. Place a check next to the current song -- the one with the asterisk (*) and hit NEXT
5. It will ask if you want to copy to "Partition 1".... hit EXECute (it takes a while, took me about 15 minutes, I think)
I'm not technically savvy enough to know why this cleared the problem up for me, but it did, so hopefully it will help someone else here. Give it a try -- I don't see how it could hurt.
By the way, I only have one song loaded in the unit at a time, which is the one I'm currently working on (I deliberately unprotected the built-in demo song and deleted it, to free up space). If you have more than one song in your machine, maybe you should copy all of them to "Partition 1" ... I don't know.
And please post if this worked for you, so others can use it... this is a common problem, as far as I can tell from Google-searching the issue.
And, by the way, always save your latest work before you try to do a mixdown -- that way, if you get this error message, you won't lose your changes. You can save easily by going to the MULTI TRACK screen, pressing and holding the POWER button to turn off the unit, and then restarting it. The machine automatically saves your changes when you shut down properly.