My external HD is asking to format

1) HD keeps asking me to format it.

http://www.tomshardware.com/answers/id-1719092/format.html

*best solution
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lonewolf7
Don't worry all your data has been fine. It's a file system error. Windows can't find out what to do with it. Try this.

Plug in the drive, bring up CMD, type chkdsk O: /f/r (O is the example drive letter yours may be different). Win will rebuild the file system and all will be fine.
If all fails than try this data recovery software.

http://www.cgsecurity.org/wiki/PhotoRec

Good luck.
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open command prompt
"C:\> My computer"  *not always appear MyComputer, it can be your name or your computer's name

then type "cd.. " to go back to root file ,type "cd.. " many time until it appears "C:"

then type "C: chkdsk O: /f " and enter
Command prompt will fix your error as below pic.

Good luck! but remember this error may happen again, so you need to get new drive to transfer your data or transfer your data another drive. 

2) My external HD is asking to format

http://www.tomshardware.com/forum/265611-32-external-format

3) My External Hard Drive is not recognized by my computer
https://www.hgst.com/support/faqs/my-external-hard-drive-not-recognized-my-computer

4) PhotoRec, Digital Picture and File Recovery

http://www.cgsecurity.org/wiki/PhotoRec