When we start the watchdog, we add the directories to be checked to a list of directories. This is added as just the filepath to the directory. For example, we might add the journal directory here. Then, the watchdog runs in a loop every x seconds, and checks the directories. As part of FCBIS, we switch the storage location. So the watchdog should get interrupted, but as part of SERVER-66444 we just continue with the loop if that happens, and the list of directories to be checked remains the same. Then FCBIS deletes the existing local files used by WT (including the journal directory) here. Then the watchdog would try to open files in the now-deleted directory, resulting in an fassert.