Time Machine backups not showing external HD files.

Thanks — these screenshots help a lot, and they actually confirm something important:

• Time Machine is creating backups successfully
• The 6TB data drive is Included
• But the WD_6TB folder inside the backup is empty in Finder, which means Time Machine is not actually capturing the content of that external drive (even though it’s supposed to).

So this is not just a “Time Machine UI glitch” — your Finder screenshot shows the WD_6TB folder exists in the backup structure but it contains no data.

Ok, so what does that mean?

• The #1 cause of this exact symptom: the external drive is not eligible for Time Machine snapshots (or Time Machine can’t read it correctly).
• This happens most often when the data drive is formatted or mounted in a way Time Machine can’t snapshot properly, including:

What are some of the common causes?

• The drive is formatted as ExFAT (very common for large external drives). If it is ExFAT, I strongly suggest using APFS instead.
• The drive has ownership/permissions weirdness
• The drive is being protected/redirected by a privacy permission issue
• The data is actually stored inside a non-standard container (i.e., Dropbox, OneDrive, some Photo libraries, etc.)
• Corruption or filesystem problems prevent enumeration

The result?

The key point: Time Machine can still create a valid backup of the Mac’s internal drive while silently failing to copy the external drive’s content.

The fact that you are seeing “3TB used” on the 8TB Time Machine disk and the “24-hour first backup” — strongly suggest Time Machine copied a lot of data, yet the “Time Machine starfield view” appears to show nothing for that external drive. To me, that points to a UI/indexing/snapshot browsing issue, not necessarily a backup failure.

Let's see if we can get this resolved. Please provide the results of the following commands in the Terminal app:

• tmutil latestbackup
• tmutil isexcluded /Volumes/<DataDrive>

Also provide a screenshot (or listing) of the backup disk’s Latest/Volumes folder.

Some questions to ponder over.

Is Spotlight indexing the 4T external drive...?

How is the 4T external HD holding your data formatted..?

Have you tried adding and removing the 4T external drive from the TM Exclude list...?

FWIW I have used CarbonCopeCloner for many years.

I run a daily CCC backup on a local external SSD at the end of each day.

Then I retrieve and run weekly CCC backup on several external drives that are stored in different location.

That answers everything except for "How is the 6T external HD holding your data formatted..?"

That “volume is shown as locked” detail is the smoking gun. If the 6TB APFS volume is locked, Time Machine can absolutely behave exactly like your screenshot: it will create the WD_6TB folder in the backup, but it will appear empty because macOS/Time Machine is not able to read the contents during the backup pass (or can only partially enumerate it).

Also, the Finder “Sharing & Permissions” you listed (Me = R/W, staff = Read Only, everyone = none) looks normal, but it does not override a locked/encrypted/ownership state at the volume level.

So, why does “Locked” breaks Time Machine (even when exclusions say Included)?

On APFS, “locked” typically means one of these is true:

• The volume is APFS Encrypted and is not unlocked early enough / reliably enough for Time Machine
• The volume has permissions/ownership disabled in a strange way
• The volume is being mounted in a restricted read mode (sometimes caused by encryption, repair-needed, or connection issues)

Here's what I suggest you do next:

Verify if the drive is encrypted or not. To do so, enter the following command in the Terminal:

• diskutil apfs list
• Look for the WD_6TB entry and see if it says: FileVault: Yes or Encrypted: Yes or mentions Locked: Yes
• If it’s encrypted/locked, the fix is usually to ensure it auto-unlocks at login and stays mounted consistently.