Wipe drives on Intel Macs for donation
How to wipe drives from several older (Intel) Macs so I can safely donate? Hardware/resources?
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If the Macs still work, you can erase the drives and prepare them for sale from Disk Utility in Recovery.
click here ➜ Use Disk Utility to erase an Intel-based Mac - Apple Support
If they don't turn on, then there is no way to actually erase the drives short of disasembling the Macs and removing the drives if they have removable drives.