empty medium; blank medium
Computing term for a storage medium that contains no data, such as a blank CD, empty USB drive, or unformatted disk.
Before writing data to an empty medium, formatting is necessary.
This blank medium has never been used yet.
空き容量 means 'free space' on a medium, while 空き媒体 refers to the medium itself being empty.
Compound of 空き (あき, 'empty') + 媒体 (ばいたい, 'medium'). The reading ばいたい is standard for 媒体 in this context.