[Verginius Rufus]

By saying there is an investigation, Comey makes his own position more secure--to remove him would be seen as an attempted cover-up. I'm not sure of the rules but apparently it is very difficult to remove the FBI director before his ten years are up...as Comey was recently gloating.

Can he be re-appointed for another 10? Maybe he thinks that if he helps destroy Trump, then the President in 2023, whether it's Hillary, or Fauxcahontas, or Corey Booker, or Kamala Harris, or Maxine Waters, will re-appoint him out of gratitude.

[Reily]

All the President has to do to remove him is write a letter saying he has lost confidence in Comey as DIRFBI.
The only issue would be handling the political blowback.