Confirm the volume and connection
In Disk Utility, make sure you are looking at the intended data volume rather than only the physical device or container. Record its format, capacity, and free space. Multiple volumes in an APFS container may share capacity.
Keep the connection stable during a scan and avoid an underpowered hub or sleep disconnect. A mechanical disk with many small files will be slower and seek continuously; that is expected.
Handle permissions and hidden directories
Backup volumes and folders created by another user may deny access. Do not recursively change permissions just to make a scan complete; doing so can damage Time Machine behavior or cross-platform compatibility.
After granting the analyzer access to removable volumes, check whether the result is complete. Hidden directories such as .Trashes and .Spotlight-V100 have system roles, so identify them before removing anything.
Identify common large consumers
Photo libraries, video footage, virtual machines, old DMGs, device backups, and duplicate project copies commonly dominate external storage. Manage a Time Machine backup through Time Machine rather than manually pruning its backup bundle.
Diskly can scan a selected folder or volume locally. Use the treemap to find the largest directory, then verify its contents and backup state in Finder before moving or deleting it.
When deletion does not return space
Check the external volume's Trash. An item deleted from that drive continues occupying that drive until Trash is emptied. Other volumes or snapshots in an APFS container can also affect shared free capacity.
After a large deletion, allow the filesystem to update and run First Aid in Disk Utility if totals remain abnormal. If the drive disconnects, slows dramatically, or reports I/O errors, stop scanning and back it up—the problem may be hardware.
Understand first, clean second
Diskly scans locally on your Mac, maps real usage in a treemap, and keeps every cleanup suggestion reviewable.
Download free trial