This looked strange to me, and I had expected, that the sharedfolder entry has to be deleted manually first. Otherwise there would be an orphaned reference in the sharedfolder entry to the mntent, wouldn't it. Therefore I asked to have a look again, if you really meant the mntent entry.
I hope this clarifies my question and my mentioning of "one level of hierarchy missing". By that I meant that I expected, down the hierarchy, before the mntent entry is deleted, the sharedfolder entry has to be deleted.
If you make a backup of config.xml before trying these things, there is very little risk. Sorry for not being clear on my explanation.
lmaxey obviously had killed the last disk he added. My died disk probably was not the last one I added (although I cannot be totally sure of that). Also I had ntfs. And I was using the root of the filesystem for sharing (which sometimes is not considered best practice).