Can't read directory with //c internal floppy drive

[tim@harlequin.com (Tim McNerney)]

I recently bought a //c which seems to mostly work.  I have successfully
booted Terrapin Logo, AppleWorks, VisiCalc, and the Pinball Contruction
Set.  AppleWorks seems to have no problems displaying a list of files on
one of its disks, but Logo ("CATALOG") and VisiCalc ("/SL") can't read the
directory from any disk.  I took the cover off the //c and the floppy drive
to see what was going on, and this is what happens:  The stepper motor
pulls the head all the way out to the edge of the disk, then it makes a
loud buzzing noise (a cross between a "braaaap" and "ra ta ta ta tat"),
then it complains of a DISK I/O ERROR.

I believe that this //c is a relatively early model, no superdrive or
anything of the sort, just a plain vanilla 5 1/4" single sided drive.

Any clues, diagnosis strategies, or repair suggestions?  If the situation
is hopeless, does anyone have a broken //c with a working floppy drive?  a
working external drive?  cheap?  I'll be happy to provide more details if
need be.  Thanks in advance.

--Tim