Windows Scheduler run Python issue - ERROR 000732

OS: Windows Server 2008 R2
Python 2.7.2
Arcgis 10.1

I have created a toolset GPS_SPEED, Model name: Speed.


Python script is very simple, as below.
DOS Command Line run successfully. No except error message.
Using Windows Task Scheduler run python, The error raised at “arcpy.Speed_TBX()”.

Erro Log:
Failed to execute. Parameters are not valid.
ERROR 000732: Join Features: Dataset Y:\Data\Current\ROADBUFFERLL.shp does not exist or is not supported
Failed to execute (Speed).

checked Y:\Data\Current\ROADBUFFERLL.shp, it is correct.

----------------------------Script------------------------------
# Import arcpy module
import arcpy

# Log file
logfile = "C:\\arcgis\\scripts\\log\\speedreturn2.log"
f = open(logfile, 'w')

arcpy.ImportToolbox(r"C:\arcgis\Tools\GPS_speed.tbx", "TBX")

try:
arcpy.Speed_TBX()
f.close()
except:
f.write(arcpy.GetMessages(2))
f.close()
----------------------------End Script------------------------------

Any suggestion are greatly appreciated.

Julie