Abstract: An optical system for an optical memory device has a sectional optical system employing an objective lens and a polariscope, e.g., a deflecting system or a galvanomirror, which drives a reflecting mirror, to change the direction of the light beam for tracking purposes. An a focal optical system is included comprising a first optical system with a positive refracting power and positioned in the optical path between the objective lens and the polariscope and a second optical system with a positive refracting power and positioned between the first optical system and the objective lens whereby the maximum light beam shift or displacement, such as caused by moving components in the optical system, e.g., the beam deflecting system or galvanomirror or the linear access actuator, is substantially zero or kept to a minimum, for example, about 0.27 mm.

Abstract: In a RAID level 5 disk drive subsystem, one or more additional disk drives, called "reserve" disks are intercoupled to the array of disk drives and a control system is included for storing recovered digital data on the additional disk drive if a sector on one of the disk drives becomes defective. The system reduces the number of disk accesses required to obtain the information stored on the defective sector. Once the information has been recovered, it is then stored on the "reserve" disk. Thereafter, whenever this information is required, the system reads the required information from the "reserve" disk, instead of going through an entire recovery process for the defective sector. Since the recovery process can take a time that is greater than several disk access times, the present invention improves the speed and the availability of data.