Comment 4 for bug 445067

The fact that it's rewriting the partition table on /dev/sda at all in this case is a bug that might be fixed either in partman-base (by making SET_FLAGS not mark the partition table as changed when the new flags are the same as the old ones) or partman-basicmethods (by not sending the SET_FLAGS command at all in the case where it won't do anything). I lean towards the latter, which is nice and simple and furthermore more efficient anyway. I think it's worth getting this fixed in 9.10, as having partition tables spuriously rewritten does tend to alarm people even if no data loss is involved.

The fact that partition boot records appear to be getting lost in the process can, I think, only be a parted bug. partman certainly isn't intentionally telling it to do this.