No matter which Mac devices (Macbook, Macbook pro, Macbook air or even iMac), users tend to encounter the circumstance that Mac OS X ejects external drive after upgrading to Yosemite 10.10. Why? And how to fix?

Fix if Yosemite ejects external drive

What makes Yosemite ejects external hard drive?

It should be logical if you track new remarkable functions (related to accessing data from external hard drive) in Yosemite compare with older version of OS X such Mavericks. I really put my eyes on Spotlight.

How to fix?

In order to attempt if Spotlight causes the problem, I turn off Spotlight on only my external drive: Connect the external drive to my Mac > access System Preferences > Click on Spotlight > Click on the Privacy tab > Drag my drive volume into the Privacy menu.
– There is another way that disable Spotlight totally by using the following command in Terminal (found in /Applications/Utilities/).
sudo launchctl unload -w /System/Library/LaunchDaemons/com.apple.metadata.mds.plist
Be noticed that the administrative password is required.

The Results

After Turning off Spotlight and restart my Macbook, Yosemite does not ejects the external drive any more. I am now able to transfer more than 250 Gb of Data without any problems.