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Abruptly unplugging an external USB hard drive without safely ejecting will easily cause serious problems, such as the external hard drive not recognized anymore or hard drive becomes inaccessible after unplugging, more seriously, all your data is currupted.

Here is risk what you need to know:
https://www.datanumen.com/blogs/will-ha ... ed-safely/

In case this happens, what can you do to rescue your data? try this:
https://www.easeus.com/storage-media-re ... gging.html