I don't understand why RAID is useless for read-only operation.

Well, maybe useless was too strong a word, but if it was my harddrives I would rather have the second one spun down than getting worn out when there is another perfectly good copy of the data on a live drive. Is there a reason to have the second drive spinning unless the first drive fails (we probably don't care about the minor increase in read performance)? So if the first drive does fail the empeg is going to crash at which point you pull it out of the dash and push it back in and it starts using the second drive instead. Granted it would make the driver more complicated.

Not to mention how I would go about finding which half failed when intially mounting the root filesystem.

Hmmm, so you are planning on doing root on RAID also? I had also assumed you where just going to mirror the music partitions since the other partitions rarely change. Again that would require a reboot if a drive crashed though. I can understand the appeal of having true RAID where the system just keeps running in the event of a drive failure, it just seems like for the empeg an offline mirror might be easier and a better use of resources.

-Mike