Prosecutors can deal with that by giving immunity. They can either give "transactional immunity" or "use immunity".

Transactional immunity, commonly known as total or blanket immunity, protects you from future prosecution for any crimes related to your testimony.

Use immunity prevents the use of your testimony or any evidence derived from that testimony against you. You can still be prosecuted for the underlying crimes if they only use evidence that they got some other way.

For example, suppose your testimony shows you are fencing stolen goods, and that comes as a complete surprise to prosecutors. With transactional immunity you could never be prosecuted for your past fencing.

With use immunity they would essentially have to ignore that they now know you are a fence. But suppose later they catch a thief, and offer the thief a plea bargain if the thief tells them how the thief is getting the goods fenced--and that thief names you. Prosecutors can then come after you for that.

I was thinking along this lines, but then he would have to declare that there is something on that encrypted drive that is unrelated but still could be used to incriminate him. That seems like an unwise declaration to make.