I bought the PN-40 when it first came out. I discovered quickly that when I disconnected it from external power the whole unit shut off and I would lose all my caches. I took it back for a new unit and the same thing happened. I went back a third time in 2 weeks (Oakland to Yarmouth) and got a refund. I've had great luck with my Garmin Oregon 400t. I still own a PN-20 that my kids use.