Can't use external encrypted HD anymore. Kernel Task over 100% cpu load.

I have a strange problem. Since yesterday I'am not able to use my external encrypted HD anymore.

Everytime I try to copy some file or even go through the folders on this disk, it "locks up" - no reaction anymore.

The kernel_task process comes up and uses over 100% CPU load and doesn't stop. I always have to force shutdown after that.

The external HD is Mac OS Extended (journaled, encrypted) Volume.

System: MacBook Air 2011, OS X 10.7.1

Any suggestions?