I’m going to need some additional information about your issue so I can help determine what is happening. What model of drive do you have? Is it the single or dual drive unit? Also, what model of drive was originally in the enclosure and what model did you use to replace the drive?
If you have a single drive unit and you replace the drive then the unit will not work. The single drive units are not user serviceable and removing the drive will make the unit unusable since the replacement drives does not have the default partition layout that the original drive has. The same applies if you try to replace both units on a dual drive My Book World or if you try to change the original for a different or larger model.