fig1 shows tape drive 100 having tape mechanism 102 , which loads and ejects tape media 104 and winds the tape media 104 forwards or backwards as required for reading and writing data . in a dds ( digital data storage ) tape drive , the read / write heads 106 are mounted on a helical scanning drum , which rotates to sweep the heads past the tape in a motion oblique to the direction of travel of the tape . tape drive apparatus 100 has eject button 108 and rf interface 110 that serves to communicate with cartridge memory 112 of tape media 104 . processor 114 of tape drive 100 serves to execute firmware 116 . firmware 116 comprises instructions 118 for providing the normal mode of operation in which the tape drive 100 behaves as a tape drive and instructions 120 for providing the disaster recovery mode of operation , in which the tape drive emulates a cd - rom drive . further processor 114 runs control program 122 which controls operation of tape drive 100 . tape drive 100 has port 124 . in the example considered here port 124 is compliant with the fibre channel ( fc ) standard . port 124 is coupled to storage area network ( san ) 126 . client computers a , b , . . . are coupled to media server computer 128 through ethernet network 130 . each one of the client computers has processor 132 for running a backup application program 134 in order to read data stored on local disk 136 and send the data over ethernet network 130 to media server 128 for the purpose of performing the data backup operation . media server 128 runs program 138 on its processor 132 for formatting data received from the client computers a , b , . . . via ethernet network 130 . during backup program 138 creates boot - images for the local disks 136 of client computers a , b , . . . and transmits the corresponding boot - image data sets via storage area network 126 to tape drive 100 . in operation tape drive 100 is initially in the normal mode of operation , i . e . tape drive 100 behaves as a tape drive at port 124 . when tape drive 100 receives boot - image data of e . g . client computer a from media server 128 via storage area network 126 at its port 124 it performs an append operation in order to store the boot - image data on tape media 104 . in other words the boot - image data of client computer a is stored on tape media 104 starting at a specific tape location ( offset ) that is identified by offset data . the offset data is stored on the tape itself or in cartridge memory 112 by means of rf interface 110 . this process can be performed repeatedly for all client computers which are coupled to media server computer 128 . each time a new boot - image is received at port 124 an append operation is performed , the boot - image data is stored on tape media 104 , and the offset data that identifies the tape location where the corresponding recording of the boot - image data starts is stored in non - volatile memory . this way multiple boot - image data can be stored on tape media 104 . this has the advantage that more efficient use can be made of tape media 104 . in order to perform a disaster recovery operation , such as for client computer a , a replacement client computer a ′ is coupled to storage area network 126 . tape drive 100 is put into the disaster recovery mode . this can be done by pressing eject button 108 when tape drive 100 is powered - on or by pressing the eject button 108 for a long time period when the tape drive 100 is already powered on . in the disaster recovery mode a cd - rom emulation is provided by instructions 120 such that tape drive 100 behaves like a cd - rom drive at port 124 . instructions 120 read the offset data stored in non - volatile memory , i . e . cartridge memory 112 or the tape itself , in order to generate a booting catalogue . on the basis of the booting catalogue the bios of the replacement client computer a ′ can select the appropriate bootable disk image . the corresponding boot - image data is read from tape media 104 and transferred to client computer a ′ via storage area network 126 . preferably the emulation performed by instructions 120 is in compliance with the ‘ el torito ’ bootable cd - rom format specification . in this instance instructions 120 generate a multiple boot - image configuration on the basis of the offset data stored in cartridge memory 112 that has a format as specified in the ‘ el torito ’ specification . fig2 illustrates an append operation of an additional boot - image received from one of the client computers . without limitation of generality it is assumed that boot - image a of client computer a is received by the tape drive from the media server computer in step 200 . in step 202 the tape drive performs an append operation and stores the boot - image a on the tape media starting at an offset o a . the corresponding offset data o a is indicative of the position where the recording of the boot - image a on the tape media starts . in step 204 the tuple consisting of the identifier a of the boot - image a of client computer a and the offset data o a is entered into the index stored on the tape media itself or on the cartridge memory . when the tape drive receives another boot image from another client computer or the same client computer , steps 200 to 204 are performed again with respect to the additional boot image . fig3 schematically shows a resulting index which maps the boot image identifiers a , b , c , . . . to respective offsets o a , o b , o c , . . . fig4 shows a flow chart for performing a restore operation , such as for the purpose of disaster recovery or migration . first the tape drive needs to be put into a cd - rom mode ( step 403 ). this can be done by pressing the eject button of the tape drive when the tape drive is powered on ( step 400 ). alternatively a command is sent from one of the client computer systems or from the media server computer to the tape drive in order to put the tape drive into the cd - rom emulation mode . for example if client computer a is replaced by client computer a ′ for the purpose of disaster recovery a command can be sent from client computer b or from the media server to the tape drive in order to start the cd - rom emulation ( step 402 ). alternatively the cd - rom mode is permanently emulated by the tape drive . in this instance an extra physical port can be coupled to the permanent cd - rom emulation or the cd - rom emulation provides a logical unit ( lun ) that is associated with a single physical port . a suitable embodiment of such a tape drive will be explained in more detail below with reference to fig5 . in step 406 the index ( cf . fig3 ) is read from non - volatile memory , e . g . from the tape media itself or from the cartridge memory . on the basis of the boot - image identifiers and respective offset data of the index a booting catalogue is provided in step 408 . preferably the booting catalogue is in compliance with the ‘ el torito ’ specification . in step 410 the bios of the replacement computer , e . g . client computer a ′, selects one of the boot images from the booting catalogue , for example in accordance with the el torito specification . the selected boot image is provided to the replacement client computer a ′ such that it can boot from the tape drive . fig5 shows a further preferred embodiment of a tape drive . elements of fig5 that correspond to elements of fig1 are designated by the same reference numerals . in contrast to the embodiment of fig1 , tape drive 100 of fig5 has firmware 140 that permanently provides the tape mode by executing instructions 118 and that permanently provides a cd - rom emulation by execution of instructions 120 . tape drive 100 has additional fibre channel port 142 . port 142 is coupled to the permanent cd - rom emulation provided by instructions 120 whereas port 124 serves for communication with tape drive 100 in the tape mode provided by instructions 118 . as a consequence it is not necessary to change the mode of operation of tape drive 100 when a disaster recovery is to be performed as tape drive 100 permanently ‘ looks ’ like a cd - rom drive at its port 142 from where the replacement computer system can boot . 100 tape drive 102 tape mechanism 104 tape media 106 heads 108 eject button 110 rf interface 112 cartridge memory 114 processor 116 firmware 118 instructions 120 instructions 122 program 124 port 126 storage area network 128 media server computer 130 ethernet network 132 processor 134 backup application program 136 local disk 138 programme 140 firmware 142 port