Hey there, @Deepankar Pathak!
I'd advise you to check the power settings on your computer as well. It might also be a good idea to check your external HDD manufacturer's knowledge base as I recall them having a good troubleshooting tutorial regarding this issue.
I'd also suggest you check for any brand-specific software utility you could use to manage your external HDD. You can easily configure the power settings from a toolkit specific for the external drive itself.
Make sure you have Indexing services
unchecked from the properties of the external HDD.
Keep me posted & Good luck!

SuperSoph_WD