I have experienced similare problem with other filesystem.
I don't know why it happens it seems that the computer can't find any info on the partions and therefore thinks it's a nonvalid dos unit.
I have several times succeeded at getting my partions back.
The only thing I did was using HDTools box and remounted the partitions as if they were free for use.
Sometimes I hade RGB backup files so I didn't have to guess the info about the partion(s).
But before I started to save RGB files I guessed and sometimes I succeeded in guessing all data right.
So I would advise all amiga users to have HD backups and alls use HDToolBox or similare programs to save the RGB structure info files on a disk and all other important info.
I don't really know how I got it to worked but my best guess is that the system somehow forgot or destroyed the parts of the partion that has all info about the partion and I simply replaced it.
