The mds program and its children can  eat  up  the  cpu  and
block  use  or unplugging of disks.  They can (sometimes) be
discouraged by:

: sudo mdutil -i off /Volumes/NAME

where NAME is the disk's mount point.  This seems to be only
partially  successful,  and the mds processes may be started
up again for no apparent reason.  This has been discussed on
the discussions.apple.com forum.