You have nothing to worry about. I have many friends on the force who are on the Juice. I just phoned one and asked him about this.
In fact there are 2 cops at my gym that cycle several times a year.
First of all one of two things:
a) Did you give your source any money? If so, maybe he doesn't have the gear and wants to keep your $ so he's having somone call you pretending to be a cop, so he wont have to pay you back.
b) If it was a real officer, then they are only trying to scare you. If they wanted you there would be no phone call no warning. You would be busted when you least expected it. (think about it.. before the cops raid a crack house, do they call first?) - They'd watch you first, present evidence to a judge, get a warrent, then bust you. This would take weeks.
Secondly:
Anything you have on your computer. Including pictures of steriods, pictures of you with steroids. Pictures of you snorting coccaine. Emails. bookmarks. ANYTHING is perfectly legal. I myself have appeared in court plenty of times as an expert witness (because of my computer related job) to know enough about this. Emails, pictures, bookmarks are NOT evidence. (unless of course its child porno etc..)
I can send emails to whom ever I want, write letters take pictures and that is no evidence because it can't be proven. It could be argued that its all lies altered pictures or there for research purposes. Think about it.. this website would not even exist if any of that was illigal.
So for gods sakes DO NOT format your hard drive(s) or get rid of your computer. They wont even touch is (unless they open your tower to see if u hid anything in there)
If you want to be safe take your gear elsewhere and do it there.
Personally I wouldn't worry about it at all. Maybe visit your source in jail to make sure he's really there and not taking your $.