Not Enough Space On Disk (Too much data?) - macOS

[dpkonofa]

I bought 2 identical SanDisk drives (4TB) that I'm attempting to have constantly mirrored. I've tried doing this with a few apps but FFS seems to be one that has the combination of options I'm looking for. Unfortunately, when I'm attempting to do the initial scan, it's telling me that source drive has 4.37TB of data and that the destination drive (which is clean formatted, same APFS file system, same options) doesn't have enough space. The source drive actually has 3.457TB used/542.44GB of free space left, according to the OS, and the new drive has 500KB used/4TB of free space left. Why is this saying that the drive has more files than its capacity? I don't have it set to use a recycle bin (it's set to delete permanently) but that shouldn't matter as this is an initial mirror to a cleanly formatted drive.

I tried searching for some more info on this but it seems like everyone else that has had a similar problem is using Windows and the 2 drives are formatting with different partition types/file systems. This is on a Mac, using APFS for both drives, and all the settings for formatting were the same for both drives. I'm stumped...

[xCSxXenon]

Using Time Machine at all? MacOS saves space by saving deltas between snapshots instead of full copies, but this means copying them does take the full space

[dpkonofa]

Do you mean on the drives in question or am I using Time Machine with any drive connected to my system? I have a dedicated Time Machine drive but it’s a completely different drive and the 2 drives that I’m mirroring are excluded in Time Machine’s settings.

[xCSxXenon]

You could run something like Disk Inventory X. It will scan the usage of the drive and show you 'geographically' where the space is being taken. Then run a comparison in FFS, then compare the two results to see where the discrepancy is. It's almost certainly a compression related issue, at my best guess, but comparing what FFS sees on the drive vs what is "actually" on it might be helpful

[dpkonofa]

Compression from what, though? The drive isn't even full and it's not being used for Time Machine at all. I don't understand why the drive would be showing in FFS that the contents are larger than its capacity.

[xCSxXenon]

Look into the other half of my message and report back

[dpkonofa]

I would love to but I don't understand what you're asking for. Also, Disk Inventory X doesn't work on macOS 26 so is there another alternative that will help me give you what you're looking for?

I guess I don't understand why setting FFS to Mirror and having Time Machine disabled for both drives would cause any compression issues or why that would show more space than FFS initially shows. I pulled up the drive in Grand Perspective (what seems to be an alternative to DIX) and total size used on the drive matches what's shown in FFS and is well below both it's own capacity (leaving 1.2 TB free) and the capacity of the 2nd drive (which is currently empty and has a capacity of 4TB).