LAst day when i was trying to access one of the folders of my external HDD explorer is telling that it cannot be accessed and need to format. how can i retrive the data from that folder? there is quite some amount of data in that folder.
Try using a data recovery software; you can find some good programs online.
Alternatively, you could send it to a specialized firm that does that, but it is more expensive.
After you get the data go ahead and re-format it.