The invention introduces a method for garbage collection, performed by a processing unit, including at least the following steps: executing instructions of a GC (garbage collection) process to direct a first access interface to read data from a storage unit, collect good data from the read data and direct the first access interface to program the good data into a spare block of the storage unit. During the GC process, each time that a timer has counted to a time period, the processing unit directs a second access interface to clock a portion of data requested by a host device out to the host device and resets the timer.