Cannot unmount and erase second hard drive.

Some computer background: MacBook Pro (15-inch, Mid 2010), 2.4 GHz Intel Core i5, 8 GB 1067 MHz DDR3

I recently upgraded my original HD to a SSD. I converted the optical drive bay into a space for a second drive and put my old laptop drive into it. I've installed Yosemite (10.10.5) onto the SSD and moved all of my files onto it. Everything works fine with the SSD and I'd like to erase the old drive and use it as a document storage device.

I've tried using Disk Utility but I get a message that the disk cannot be unmounted. I've tried to use the Terminal to force it to unmount, this failed. I've tried using Command + R then the Disk Utility but the partition cannot be unmounted. I think it may have something to do with the recovery partition on the drive but I don't know why that would mean I can't reformat the old partition.

Here are the drive details from the System Information:

Hitachi HTS545032B9SA02:

Capacity: 320.07 GB (320,072,933,376 bytes)

Model: Hitachi HTS545032B9SA02

Revision: PB3AC60W

Serial Number: 100710PBSC00EYHHX1GS

Native Command Queuing: Yes

Queue Depth: 32

Removable Media: No

Detachable Drive: No

BSD Name: disk1

Rotational Rate: 5400

Medium Type: Rotational

Partition Map Type: GPT (GUID Partition Table)

S.M.A.R.T. status: Verified

Volumes:

EFI:

Capacity: 209.7 MB (209,715,200 bytes)

BSD Name: disk1s1

Content: EFI

Volume UUID: BDC1974F-6B8C-3DAE-9DB2-3AA3C17BF506

disk1s2:

Capacity: 319.21 GB (319,213,174,784 bytes)

BSD Name: disk1s2

Content: Apple_CoreStorage

Recovery HD:

Capacity: 650 MB (650,002,432 bytes)

BSD Name: disk1s3

Content: Apple_Boot

Volume UUID: 76AC3AC2-C526-37EC-997A-346CA562B424

There is no data on the drive I want to keep, how do I erase it?

Thank you