1. A data storage apparatus comprising: a storage medium; a flash memory buffer configured to store write data to be written in the storage medium; and a controller configured to regulate a data transfer operation between the flash memory buffer and the storage medium, wherein the controller compares the amount of unused space, as a percentage of a whole storage space of the flash memory buffer, in the flash memory buffer to a first reference value and compares the amount of valid data in the flash memory buffer to a second reference value, and in response to the comparisons, conducts either a block reclaim operation on the flash memory buffer or a buffer flush operation to transfer valid data from the flash memory buffer to the storage medium, wherein, when the controller receives an external request for a data transfer operation during the block reclaim operation, the controller suspends the block reclaim operation to conduct the requested data transfer operation, and wherein the controller resumes the suspended block reclaim operation if an additional request for a data transfer operation is not received during a predetermined time.