"Mr. Comey wanted more personnel and resources to devote to his examination of the Russia ties, and the White House obviously wanted fewer of each. "

That appears to be the heart of this matter. 
Whatever we might think of Comey, we should assume that FBI investigators are a competent and reasonably principled bunch. If they smell a rat, they will pursue it. If they discover that there are more rats than they thought, they will ask for more resources.

If Mr. McCabe is a principled guy the FBI investigation should proceed nicely. If he isn't and tries to suppress it we may see leaks and other action from front-line investigators who refuse to bury what they've discovered.