Formatting 2nd internal drive?

Important dates to keep in mind about the age of the Macs*:

http://www.macmaps.com/docs/DOC-13695/

If your Mac is older than March 15, 2010, you can get

Snow Leopard Retail 10.6.3 which looks like the image above and does not say Upgrade, Dropin, or OEM and then upgrade to 10.6.8 combo.

If it is between March 15, 2010 and July 21, 2011 you have to find the prebundled 10.6 installer, or a hard drive with a bootable copy of the operating system to clone over to the Mac, as long as it is compatible according to the first link.

July 22, 2011 and later Macs can install through command-option-shift-R internet restore the oldest system that came with them, upgrade to 10.12.6 directly from that oldest system as long as they are older than 10.12. I would not try to install 10.11.6 or earlier on a Mac newer than this date if it is not apparent in this install option, since Apple's installer was designated for 10.6.8. You need to use Disk Utility to reformat the drive, if something newer is installed, and you need the older operating system that came with the Mac.

Agreed a 10.10 eBay install is a waste of money, and could be compromised software in some manner since Apple never distributed it on its own.

*Links to my pages may give me compensation.

When you installed Mac OS X Yosemite it would have installed the Recovery HD also, it is by default hidden.

If you want to check, open Terminal (Applications/ Utilities) and type in the following text.

diskutil list

press Return.

So if you don't need an OS on it or any further partitions then just format it as

described above and that is you done.

Why are you buying a Yosemite install disk from e-bay it only ever came as a download.

It is a free download from Apple,

How to upgrade to OS X Yosemite – Apple Support

go to Section 4 and click on Get OS X Yosemite

this will download Install MacOSX.dmg to your downloads folder.

(The next section can only be done on a mac that is capable of running Yosemite,

a mac that came preinstalled with an OS later than Yosemite will refuse to do the next bit.)

When downloaded open to InstallMacOSX.pkg, double-click on

that and an installation window will open, this does not install Yosemite

but converts the InstallMacOSX.pkg to the Install OS X Yosemite.app which 

you will find in your Applications folder.

 (If the installation window asks which disk you want to install to, you must pick 

the disk that you are booted to at the time. Not any internal or external disk that 

you want to eventually install Yosemite on, that is for later.)

It is also possible to create a bootable USB installer disk using the Install OS X Yosemite.app

in the Applications folder and the createinstallmedia command in the Terminal app. 

Read the instructions here,

How to create a bootable installer for macOS – Apple Support

sudo /Applications/Install\ OS\ X\ Yosemite.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ Yosemite.app

Or use this free app, https://macdaddy.io/install-disk-creator/

Caution! This work is generally best done by an authorized service professional, and only after making two separate backups from the computer. http://www.ifixit.com/ provides extensive direction if you are out of warranty and confident you won't make a mistake.

SD cards are generally much slower than the interface for the built-in hard drive. With the iMac be sure to not forget to put the heat sensor foam on the new drive after removing it from the old drive. And be sure not to lose screws attaching the internal display. I found it useful to wrap my screws on the screwdriver with tape, to avoid the magnets from pulling them off the screwdriver. If you are using a model with VMI adhesive strips for the screen, be sure to get the right one for the machine, as they are not reusable. Be sure as always to use anti-static measures such as a grounded workbench and wriststrap. The LVDI cable can easily come unhooked when reattaching the display, and will leave the machine booting with loud fan if not attached back properly.

10.10 will not run on the SSD unless you force format it Mac OS X Extended Journalled. Note, 10.10 can't see 10.13 and later APFS formatted drives, which will happen automatically when you upgrade to the later operating systems, if your machine supports them. Any system that can run 10.10, can also run 10.11. The installer for 10.11 though runs best when booted off 10.6.8.

If you are running an early 2011 or earlier model, you'll need 10.6 original installer discs if you need to install 10.6 on its own partition. And an early 2010 or earlier version can run 10.6.3 retail, and use the 10.6.8 combo updater to get the App Store.

If you are going to install another disk in your iMac you will need to format it as

Mac OS Extended (Journaled) on a GUID Partition Map using Disk Utility.

You can then just use the whole disk as one large storage space.

Or you can split it into different partitions for different storage needs.

Do you want to install Mac OS X Yosemite on this new internal disk.