I'd do a couple of things differently from how Jon did:

for i = 67 to 90
   x = chr(i)
   if inlist(drivetype(x),3,4)
      * Make sure the drive is ready (type 4 is either removeable
        or a network drive)
      * Put your search code here
   endif
endfor

The important thing is to use the CHR() of the variable in the loop.