A removable hard disk drive (HDD) bay has a casing, a door assembly and an extracting assembly. The door assembly has a pivoting member, a cover slidably mounted on the pivoting member, a retainer slidably mounted in the cover and selectively hooking the casing, and a driving member driving the retainer. The extracting assembly pivotally connects the door assembly to the casing. When the door assembly is opened relative to the casing, the extracting assembly extracts the HDD (AA) with a simple mechanism and the door assembly does not occupy any room in the casing. The casing does not have to reserve a room for the door assembly and a volume of the removable HDD bay is efficiently minimized.