now if one drive is missing or broken, why should the other one mount when it is not possible to write to it?
I Think we are getting confused here, as far as a RAID 1 setup when a single drive fails the RAID goes into a "degraded" state, the Good drive should mount without issue, the Bad obviously will give all sorts of problems
you might want to switch the controller to jbod to access your data, that should always work. I would still remove the broken drive though...
OK, as far as changing settings from RAID to JBOD (Just a bunch of disks), I do not recommend this, some controllers will zero a drive is you mess with the RAID info, so DO NOT EVER switch a drive's mode if you want the data intact, most Intel Embedded RAID setups will simply remove the "RAID definition" and NOT reformat the drive, so that is a rare exception to the rule; Again, most mirrored (RAID 1) implementations are meant to Mount and work and simply message the user that a drive has failed...
Back to the Ext case thing, since that is the context we were talking about... NEVER change dip switches or redefine RAID via dials or switches (if you care about the data) since many external boxes that do there own hardware RAID implementation will "nuke" the partition table and basically reformat the drive when changing the RAID definition 0,1,JBOD(no raid). I can tell you that drives removed from most Firewire RAID ext cases/products I have used over the years as far a OWC, Lacie, and other mac vendors are totally readable if you put the drive on an internal controller.
Some NAS products, like some of the fucking western digital boxes (and seagate boxes) utilize "encryption" on their circuit boards to the point that all drives show up as "unformatted" if you take them out of the case... a real bummer, to the point you have to get (from china) the exact revision on the Backplane/controller board is you get a fried case...which happens more than you think; even cases that look identical and may be the exact model will not read the drives, if the board rev in different.... so, I HATE Propriety NAS boxes for this reason, you better have an extra case (exact model) around if your NAS logic board fries