Disk space usage

I have the 4TB NAS - do need to leave some space as overhead so the NAS can do disk clean up?  If so, how much is recommended - 25%?  Or can the full usable space be utilized, which is ~ 3.8 TB effective.

Hello and welcome to the WD Community.

When you say a clean up, are you referring to a reset or factory restore?

He probably means defragment.

In which case, look up ‘ext4 defragmentation’.