Apple - USB Drive Will Not Mount, Not Listed in Disk Utilities, but Found In System Profiler

I just started having this exact same problem with my FreeAgent Go drive which has always been working perfectly. Like the original poster I cannot see the drive in Disk Utility but it does show up in System Information. After a little more digging I found that the OS is trying to repair the volume BEFORE letting Disk Utility see it via an process named "fsck_hfs". When I force quit that process in Activity Monitor (it's owned by Root so you'll need to give it permission), the volume immediately appeared in Disk Utility. I'm running a repair on it now - fingers crossed. My guess is that since this is formatted HFS+ Journaled something about the journal consistency got flagged and the OS is attempting to repair it from the journal. I'm shocked that they hide it from Disk Utility while it's happening though - much better would have been to display the drive there without mounting the partition, and display a message that this is being done. Oh well at least I can see why Windows would mount it but OSX won't now (any OSX machine would start performing the same repair process).

The problem was VirtualBox. Specifically, the Windows 7 VM instance I had running was "grabbing" the device for itself. So, it unmounted it from OS X and passed the USB port to the VM. It was doing it automatically so I couldn't see anything.

To fix, you uncheck the device next to the USB filter.

The filter automatically connects a USB device to the VM instance so you don't have to do it manually (like my Zune).

Now, this is where it becomes confusing...I have 4 VMs running and all are set to autoboot when the host (OS X) starts. I haven't changed the settings in any of those VMs in at least 2 months and the the Windows 7 machine in at least a year. I don't need to; as everything was working. Now, way back when I was configuring them, I did attach the USB drive to the VM so I could install programs, backup some files etc. But when I was done, they were disabled. Somehow, they got re-enabled.

For my case:

• The device doesn't show in Disk Utility
• It does show in System Information under USB
• It works in other computers
• Other USB sticks work in this computer
• Changing USB ports makes no difference

I just rebooted the system, then it's mounted and working correctly.