Having spent an unhappy afternoon with one of these devices I guess the way to open it is to crack the seal that joins the two case halves then prise them apart. There are no screws, or any obvious way to get in, that I have found. It seems like you have done a pretty good number on it already so any further damage will be minimal. However, I can't see what help cleaning the sand will do: there are no moving parts for sand to get jammed into (it is a battery, a hard drive and some electronics) so I would be tempted to leave it alone. The question is: what do you do if the rechargable battery needs replacing?