We are talking her about a known grub bug, well at least in Arch
The savefault means that when you boot into Windows for example, the next time the default booted OS will be Windows again.
It is the savedefault flag that causes the bug, the "file not found error 15" and also "filesystem unknown type 0x7".
It can be solved by doing as root in terminal:
#grub-set-default 0
