Moving Home Folder Back to Macintosh HD
Here's what I would do:


Backup your full disk (all partitions) to an external disk using Carbon Copy Cloner or SuperDuper.
Copy your home folder back to the main partition (to /Users) using "Paste Exactly": in Finder, go to your home folder on /Volumes/Data HD, select Edit &rarr; Copy from the menu, go to /Users (still in Finder), open the Edit menu, hold down the Command, Option and Shift modifier keys, and select "Paste Exactly".
Go to System Preferences and open the "Users &amp; Groups" preferences
Unlock using the lock-icon in the bottom left
Right-click the user account of which you're moving the home folder
Choose "Advanced Options…"
Change the user's home directory using the "Choose…" button and point it to the copy you just made in /Users
Reboot your computer


Extra check:


Rename old the user folder in /Volumes/Data HD so that you are sure OS X cannot find it anymore
Reboot your computer
Make sure that everything still works


If everything still works, you can go to Disk Utility, delete the partition corresponding to /Volumes/Data HD, and then resize the main partition to use the entire harddisk. All this is done by selecting the disk on the left-hand side in Disk Utility (so the main entry that is listed above the two partitions, not the partition itself), and then selecting the "Partition" tab. Select the partition you want to delete, click the 'minus' button, click Apply. Then resize the first partition by dragging its bottom border down to cover the entire drive and click Apply again.

For more information and/or alternatives, please refer to this post: https://discussions.apple.com/message/23510349#23510349.