What is .indexdirectory file?
The ".indexdirectory" file extension is related to Spotlight, an indexing process of Mac OS X.
Whenever an USB flash disk is plugged to a Mac, Spotlights creates two hidden folders .trashes and .spotlight-V100 on it.
An *.indexDirectory file is created inside the *.spotlight-V100 folder. These files are most likely used only for indexing purposes.
Recommended Software Programs to open .indexdirectory file