"Can’t reinstall MacOs because disk is locked (2nd hand MacBook)"に関する回答1.) Open the terminal from the reset mode. 2.) Type this command in: diskutil list and hit enter. 3.) If there are no errors, type this command: diskutil unmount force /dev/disk0s2 and hit enter 4.) You will now be able to look at all of the actions through Disk Repair (erase/partition) 5.) Select Partition and create a new partition (select OSX Journaled) 6.) Hit continue and if it works then you can go to download your operating system and your disk will see two disks. Select the disk that is no longer locked.