Before you go further, it's better to know what you can do and can't do. Do not go straight to reformat this not-mounted external hard drive unless you've restored data from it or backed up it before. Reformatting will wipe the data on this disk and cause serious data loss. Do not run First Aid to directly repair the disk if you didn't restore data from it or back up to it. Any further operation is very likely to overwrite your original files and make them unrecoverable. Check if you have a copy of a data backup. If not, recover important files from this unmountable external hard drive first to avoid data loss. I used a different computer to do this. I found some suggestions on the LaCie support site. Some Seagate and LaCie software use a kernel extension (kext). Apple silicon Macs require that users change the security settings to ‘Reduced Security’ for the software to function properly. To change the level of security on your startup disk: Shut down the Mac Press and hold the power button until you see “Loading startup options” Click Options Click Continue If asked, select a user Click Next Enter password for admin account In the menu bar at the top, Select Utilities Startup Security Utility Select the startup disk If the disk is encrypted, Click Unlock Enter Password Click Unlock Click Security Policy Select Reduced Security Put a check next to “Allow user management of kernel extensions from identified developers” Click OK Enter admin password click OK Once changes are applied, click the Apple icon and Restart