If you by mistake deleted files from external HDD, do not worry, you can recover deleted files from external hard drive with professional data recovery software. See this step by step guide
How to recover deleted files from external hard drive
The external HDD data recovery software is easy to use. Just few mouse clicks and you can recover deleted and formatted files such as photos, videos, songs, documents, and other files from external hard drive. You can restore all deleted files, including png, jpg, jpeg, gif, bmp, tif, tiff, raw image data, mp4, mov, wmv, asf, mkv, avchd, pdf, docx, pptx, and other files from external hard disk, such as Seagate backup plus, Western Digital my passport, WD My Book, Western Digital Element, Transcend StoreJet, Seagate Expansion, G-Technology G-Drive Mobile, Toshiba Canvio Advance, Lacie Rugged, Lenovo Portable, Seagate Backup Plus, etc.
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Sandisk file recovery freeware - recover deleted formatted photo video files from Sandisk usb flash drive memory cards 