Are the disks that you intend to convert to 'spare' part of an aggregate ? If yes - You must destroy the aggregate, all the disks will be converted to 'spare'. Before that : Make sure the aggregate does not contain volumes (data) that are or will be neded in future. If so, then first move those volumes to another aggregate.
Steps to convert disks as 'spares' for sanitization purpose: [Objective : To erase all data & destory filer]
1) Reboot the filer and press ctl+c (holding control key and C key together) early in the boot process where it says (in effect) type ^C to get the boot options menu.
2) From the menu select item 4) Initialize
3) After you have selected option 4, you should be prompted by
Zero disks and install a new filesystem? Answer "y" This will erase all the data on the disks, are you sure? Answer "y"
4) Once the disks are erased, filer will begin to boot again, when prompted, enter Ctrl + C for the special boot menu. Set up the root volume aggregate.
4a) choose Option 4a from the special boot menu. Option 4a is only available if running Data ONTAP 7G or later.
5) Once filer boots up (set it up with minimum config) and have you gained access to console (filer>), you can then begin the 'sanitizaiton process' on the spare-disks as per the kb article shared earlier.