ReadyNAS Duo RND2000 several questions of not starting up, Raid0 and removing one disc....

EDIT-JennC beat me, so there is some overlap in our responses.

-You'll need to risk pulling the plug if it won't shut down.

-You say you are running RAID-0 - do you have two data volumes  (C and D) or just one (C)?  If you do only have one volume with RAID-0, a disk failure means you've lost the complete disk volume. Please let us know how many volumes (and what firmware the NAS was running).  

-A clicking sound is almost certainly a bad disk.  The only other option would be a fan bearing.  You can rule out the fan bearing by starting the NAS with both disks removed.  

-RAIDar 4.3.8 is available for OSX - you can find the download here: http://www.downloads.netgear.com/files/ReadyNAS/RAIDar_Mac_4_3_8.dmg

Assuming a bad disk - and only one data volume (and no backup):

You could use a data recovery service to try to get your data back.  That would be expensive (usually the cost depends on how much labor is needed).  Netgear offers data recovery, there are other reliable services.  Seagate offers their own service, Western Digital lists some services on their web site.  There are some scams out there, so be careful if you pick a different service.

You could also purchase a RAID recovery software package.  These are also generally quite expensive.

The third option is to attempt to "clone' the bad drive - using a cloning program to copy all the readable sectors to a new disk.  If you want to keep your other data recovery options open, clone both drives.  Then install the clones in the NAS (in the right order) with the NAS powered down, and see if it boots.  There usually is still data corruption, but you might get the bulk of the data back.

None of these methods are certain (and if the disk has totally failed, you've certainly lost your data).  You might begin by setting a ceiling on how much money and effort you are prepared to spend to get the data back.