The Legend of Zelda: Skyward Sword HD has 24 Heart Pieces to be found in total. For every four you collect, you'll increase how much life you have by one Heart.
There are three Heart Pieces in Eldin Volcano. Here's where you'll find them.
First Heart Piece - Eldin Volcano
- Location: On a platform, at the foot of the volcano
This fragment is located east of the foot of the volcano, on a high platform not far from the skeleton bridge.
Second Heart Piece - Eldin Volcano
- Location: Fire Sanctuary
- Prerequisite: Obtain the Mogma Mitts
In the northwest of the Fire Sanctuary, you'll come across a pile of dirt. Have at it with your Mogma Mitts.
Inside you'll need to flick a switch to open a gate. In the burrow beyond you'll have to catch the Mogma in order to obtain the Heart Piece.
Third Heart Piece - Eldin Volcano
- Location: Volcano Summit
- Prerequisite: Obtain the Mogma Mitts
Head to the entrance to the Fire Sanctuary, where you'll find three piles of dirt that you can dig up using the Mogma Mitts.
The one with butterflies above it is the one you want, which will lead to a passage containing the Heart Piece.
Be mndful that some Heart Pieces can only be found later in the game, once certain requirements are met or equipment is obtained. Feel free to come back later!