fseventsd is at 100%CPU when time machine on external hard drive is connected
This solution worked for me:

In Terminal, make a directory called ".fseventsd" (mkdir .fseventsd) in the drive mounted by Time Machine, and add a file inside called "no_log" (touch no_log).

It tells fseventsd process not to bother reading the backup drive.

The annoyance ceased immediately. Time Machine backup worked flawlessly.

I found the solution on the MacRumors forums.