1. A hybrid drive comprising: a disk; a head actuated over the disk; a non-volatile semiconductor memory (NVSM); and control circuitry operable to: generate a NVSM command queue operable to store access commands for the NVSM; generate a disk command queue operable to store access commands for the disk; estimate a first execution time needed to execute the commands in the NVSM command queue; estimate a second execution time needed to execute the commands in the disk command queue; insert an access command into a selected one of the NVSM command queue and the disk command queue in response to the first and second execution times; update one of the first and second execution times in response to an estimated execution time of the access command; execute a first pending access command stored in the disk command queue; and when an actual execution time of the first pending access command is longer than an estimated execution time of the first pending access command, transfer a second pending access command from the disk command queue to the NVSM command queue.