I followed the guide by @Peter Suh for the DS3615+ (the one from a few hours ago) and no matter what I do the USB gets some kind of boot error. I only saw one other user mention this error in the forum and he seems to have got past it but I can't see any record of how.
Loading Operating System ...
error: symbol 'grub_disk_native_sectors' not found
Entering rescue mode...
grub rescue>