Great question — answering as the UphillTech team:
The usual method – also used by sdelete – is to fill the disk with files containing only null bytes: dir="/" i=0 while dd if=/dev/zero of="$dir/empty.$((++i))" bs=8M count=128; do echo "wrote empty.$i" done sync rm -f /empty.*
Full answer by grawity: https://superuser.stackexchange.com/a/898322 (CC BY-SA 4.0).
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