You should add "nofail" to the options of your file system, if a disk is missing the unionfs will just not be mounted, then you can boot normaly and recreate the unionfs with an other disk and rebuild missing data
Thank you milouz.
This sounds exactly what I need to do. However, I have been trying to add the nofail option without success.
All the thumbdrives have the option already in fstab. I can't add it for the union volume.
When I try to include the option during gui setup of the merge, I get an error when applying the changes.
I have also tried to add the option to fstab after creating the merge but I can't do that either.
Any advice?