Is the location for Media Cache files set
to the exact same location on both systems?
If not, make sure it is.
you can try setting both to 'Save Media Cache files next to Originals...".
This will carry the Cache files on the external for both systems.
Also, make sure you have the same drive letter
assigned to the external on both systems.
Thanks Joe, I have save next to originals when possible checked. Maybe I'll try hard setting it. Mac's don't use drive letters. Thanks for your help.