I have a MacBook Pro (Retina, 15-inch, Mid 2014). I am running macOS Sierra 10.12.4 . Is there any way to find out if I have a hard disk or a solid state disk? (I have a total of 250 GB of storage if that is necessary)
All retina Macbook Pros have PCIe SSDs. Previous MBPs (2012 + earlier) shipped with HDDs.
To find this out, find your model information by going to Apple (top left corner of the screen) -> About this Mac. Find the year (Mid 2014), then use either Apple's website or a website such as everymac.com to determine the specifications of the storage originally shipped with the computer.
Go to "About this Mac" on the top left of the screen in the Apple menu.
Click on the Storage tab. This shows all attached drives.
For SSD it shows as "Flash Storage"
Go to About this Mac under the apple logo on the left corner. Under Overview tab, click on "System Report...". This will open another window with "Hardware" selected on the left side of the column. Click on "Storage" and then on the right it will show "Medium Type" as SSD.