TimeMachine keeps doing Full backups of SDXC drive

Summary:

I have a macbook air 2015 with an internal SSD and an external 200GB Flashdrive (SDXC slot)

TimeMachine does full backup of SDXC flash drive every time it runs. It does a normal incremental backup of the internal drive.

problem is unaffected by reformatting disks or changing the disk format from HFS+ to APFS. I have observed this with different, initially blank, time machine disks.

100% reproducible.

Steps to Reproduce:

Run time timemachine multple times with an SDXC card filled with data. The SDXC card has "ignore permissions" switched off (i.e. it respects permissions and owners) but I don't know if that is required.

Expected Results:

Should get incremental backup of only changed files on both internal disk and external SDXC disk.

Actual Results:

Internal disk backup is incremental as expected but the SDXC is fully backed up ever run of time machine.

Version/Build:

10.13.1 High Sierra

Configuration:

2015 macbook air. 500GB SSD internal, 200GB Samsung SDXC card external.

Regression:

here's what I've done to try to resolve this.

1. Erase and reformat the SDXC card (both HFS+ and APFS. Both MBR and GIUD)

2. Reinstall the OS

3. Verify the backup disk and SDXC disk

4. Change to an entirely new (never used) Backup disk.

In all cases the backup disk I'm using is formatted HSF+ and is a reliable working disk.

To diagnose this I have looked this using

1. tmutil compare

2. time-dog

3. Rsync.

If I run Rsync twice in a row, both times copying the SDXC disk to a target disk, then the first time it copies all the files like you would expect and the second time it copies no files because nothing changed, as you expect.

So Rsync does not detect any changed files.

if I run tmutil compare it actually tells me there's no large changes between the backups! it will report a few hundred megabytes (plausible) of changes instead of 130GB.

so tmutil is not working correctly because there really were 130 GB of file transfers. I suspect this points to a data base error if it's relying on a data base to determine what change

Time-dog is a free git hub perl script that compares any two backups to measure what changed by looking at the inodes. (two files linked by a hard link share the same inode so you can tell they did not change) It does detect that there are 130GB of new files. SO it's reporting the truth. The fact that it does not rely on a database to determine the changed files makes it reliable. This is my guess about why tmutil compare is failing.

So for now I cannot use time machine anymore.

there are other people on apples support forums reporting this too but it seems to be for older OS than High sierra.

The only common feature here that I can think of is the SDXC card itself. I have reformatted it but it's always the same card with the same data placed on it.