of fig4 the frequency of any data transfer size may be not higher than a predetermined level since the frequency distribution is relatively uniform in this case the disk drive of the present embodiment may set the largest data transfer size 401 as the basic data transfer size in the disk drive of the present embodiment it is also possible to choose the largest data transfer size as the basic data transfer size from the most frequent data transfer sizes for example the basic data transfer size may be chosen from the three most frequent data transfer sizes if the issued commands are distributed as in fig3 the largest data transfer size 128 sectors 303 is chosen as the basic data transfer size from the three most frequent data transfer sizes 124 sectors 301 126 sectors 302 and 128 sectors 303 referring back to step 209 if the amount of data read so far in advance is smaller than a multiple of the basic data transfer size the read ahead is continued until the amount of data reaches the multiple of the basic data transfer size step 210 then processing of the received command is started step 211 if the amount of data read so far in advance is equal to or larger than the multiple of the basic data transfer size the read ahead is immediately stopped to start processing of the received command step 211 in the flows of read ahead control processing shown in fig5 an av data section is secured.