Here's the hardware: Late 2009 iMac 27" i7 2.8GHz 8GB RAM 1TB HDD Seagate running OS X 10.6.7 Problem external drive: Western Digital My Book Studio II - 4 TB (2 x 2 TB) USB 2.0/FireWire 800/400/eSATA Desktop External Hard Drive. The WD drive is configured in RAID1, and about 40% full (i.e, there's about 900GB on it). It is connected to the iMac through USB. I have a number of drives plugged into the iMac including a few more WD My Book drives. I don't shut the iMac overnight. This morning, I saw one of the external drives (the 4 TB) was not showing on the desktop. I can't get it to show. Here's what I did: 1)I repaired permissions a few times 2)rebooted a few times 3)I disconnected the USB cable on both the WD and on the iMac, and reconnected several times. 4)I tried all the USB ports on the iMac. So it is NOT a bad port/ports. 5)I tried a different new USB cable. So it is NOT a bad USB cable. 6)I disconnected the USB cable completely, and connected the WD to the iMac through a FW cable and the FireWire800 port. So it is not a problem with the USB interface on either the WD drive or the iMac. 7)connected the WD drive to my iBook running 10.4.11 - same result. So it's probably NOT a problem with the iMac. Throughout all this, all the other drives show up, but not this one. There appears nothing wrong with it, externally, i.e. the drives spin up, and the LED lights show activity when I plug the USB cable to the iMac and pull it out again. Here's what it looks like: I can see it in Disc Utility, though it doesn't show the name I gave it, instead it has a generic 2TB WD My Book Media and disk5s2 name under it. When I select "info" it shows as USB connection bus etc. I also have the WD Drive Manager installed. The drive appears there as well, but shows as Used Space 0% which is wrong (actually it's some 45%). Temperature: OK and RAID Status: Healthy. I launch the manager, and it shows the following info: "RAID Setting: RAID 1 RAID Status: Healthy Disk A Status: Good Disk B Status: Good" It is exactly the same situation and the same readings when I hook it up to my iBook. So: Why is it not showing on the desktop (Finder), and why is it showing as 0% used in WD Drive Manager? And: How do I get this to work? I have some very important data on it - that's why I had it in RAID1. Is the enclosure toast? Is it the RAID controller? What next? Can I just extract one drive, put it in a different enclosure and get the data off it? What do I do? Thanks!