Making players follow MY plan...subtly, Psychology experts welcome Options

[Sengir]

So, I'm busy procrastinating my thesis by thinking about an adventure I'd like to run...but now I'm stuck there, too (IMG:style_emoticons/default/biggrin.gif)

The setup is as follows:
- Players are hired for a standard B&E job
- The objective is to break into a server room, find server XY, and hot-swap a random harddisk of that server for one provided by Mr. Johnson.
- The original disk is completely irrelevant as far as Johnson is concerned, he just hires the players to slip his disk into the system

But as fate would have it, seconds before the players do the swap, a corp exec suffers from a nasty case of exitus letalis. And by sheer dumb luck, the disk the players pulled out contains the security footage. Of course the players don't know about the assassination until later, and even less do they know that the random HDD which had to make room for their cuckoo's egg is actually important.
And that is where my problem lies: How do I make sure they take that damn disk with them, without spoilers or blatant railroading? Best thing I came up with so far is dragging along an NPC who does the swap. But we've rarely used NPCs so far, so I'm still looking for a more elegant solution...any suggestions?