Why not use the uC to implement a secret compartment on the drive instead? Like switch between two flash chips.
When the cops plug in the drive, they see the content you want them to see. And there are no hidden partitions or anything: your uC passes through all signals to a complete storage device. They can examine the partitions at the block level, and all that; nothing is hidden.
You have some mechanism to instruct the uC to switch different storage device (which, of course, is encrypted so that it's protected should it be discovered).
When the cops plug in the drive, they see the content you want them to see. And there are no hidden partitions or anything: your uC passes through all signals to a complete storage device. They can examine the partitions at the block level, and all that; nothing is hidden.
You have some mechanism to instruct the uC to switch different storage device (which, of course, is encrypted so that it's protected should it be discovered).