6 hours for diskpart clean all on a 1TB HDD over SATA is very believable. You may see "1TB" and expect modern speeds, but clean all is basically the slowest possible operation you can do to a spinning disk because it writes zeros across every single sector on the drive. Unlike normal clean, which just wipes partition metadata in seconds, clean all performs a full sequential overwrite.
Real-world HDD speeds matter here more than theoretical specs:
5400RPM HDD → often 50-90 MB/s sustained
Older/slower 7200RPM drives → maybe 80-150 MB/s at best
Speeds also drop as the write head moves toward inner tracks
If your average write speed during the process was around 45-55 MB/s, then:
1TB ÷ 50 MB/s ≈ roughly 5-6 hours
So, from a pure math standpoint, your timing is not suspicious at all.
