A drive bay is a standard area for adding hardware to a computer. Most drive bays are fixed to the inside of a case, but some can be removed.
StarTech.com 2.5

StarTech.com 2.5" SSD/HDD Mounting Bracket for 3.5" Drive Bay - Tool-less Installation

StarTech.com
£9.99
StarTech.com 3.5in Universal Hard Drive Mounting Bracket Adapter for 5.25in Bay - TAA

StarTech.com 3.5in Universal Hard Drive Mounting Bracket Adapter for 5.25in Bay - TAA

StarTech.com
£6.99
StarTech.com 3.5

StarTech.com 3.5" to 5.25" Front Bay Mounting Bracket

StarTech.com
£9.99
StarTech.com 2.5

StarTech.com 2.5" to 3.5" Hard Drive Adapter - For SATA and SAS SSDs/HDDs

StarTech.com
£27.99
StarTech.com Dual 2.5

StarTech.com Dual 2.5" to 3.5" HDD Bracket for SATA Hard Drives - 2 Drive 2.5" to 3.5" Bracket for Mounting Bay - TAA

StarTech.com
£14.99
StarTech.com 2.5in SATA/SAS SSD/HDD to 3.5in SATA Hard Drive Converter

StarTech.com 2.5in SATA/SAS SSD/HDD to 3.5in SATA Hard Drive Converter

StarTech.com
£16.99