Simple: Exfat does not support symbolic links. So every file that’s just a symbolic link on your btrfs filesystem is getting copied in full (the link is being resolved) to your Exfat drive.

Solution don’t use Exfat. For backups from btrfs, I recommend using btrfs with compression enabled.

Also don’t forget to rebalance your btrfs partitions regularly to reclaim lost space! Also, delete old snapshots!

The simplest explanation for the size difference could be if you have a symlink in your home folder pointing outside it. Idk if rsync traverses symlinks and filesystems by default, i.e. goes into linked folders instead of just copying the link, but you might want to check that. Note also that exFAT doesn’t support symlinks, dunno what rsync does in that case.

It would be useful to run ls -R >file.txt in both the source and target directories and diff the files to see if the directory structure changed. (The -l option would report many changes, since exFAT doesn’t support Unix permissions either.)

As others mentioned, if you have hardlinks in the source, they could be copied multiple times to the target, particularly since exFAT, again, doesn’t have hardlinks. But the primary source of hardlinks in normal usage would probably be git, which employs them to compact its structures, and I doubt it that you have >300 GB of git repositories.

rsync does not delete files at the target by default, it has kept all of the original files when they were deleted from the original source location.

You must specify --delete for it to also delete files at the target location when they are deleted at the source.

Let’s back up and check your assumptions: How did you check that the disk usage of your home folder is 385GiB and that there are 780GiB of free disk space on your external drive?

BTRFS supports compression and deduplication, so the actual disk space used might be less than the total size of your home directory. I’d run du -sh --apparent-size /home/sbird to check how large your home dir actually is. If it’s larger than 780 GiB, there’s your problem. Otherwise there might be hardlinks which rsync is copying multiple times; add the -H flag to copy hardlinks as hardlinks.

There might be a possibility that recursion is happening and a directory is looping into itself and filling up your storage.

I have some suggestions for your command to help make a more consistent experience with rsync.

1: --dry-run (-n) is great for troubleshooting issues. It performs a fake transfer so you can sort issues before moving any data. Remove this option when you are confident about making changes.

2: --verbose --human-readable (-vh) will give you visual feedback so you can see what is happening. Combine this with --dry-run so you get a full picture of what rsync will attempt to do before any changes are made.

3: --compress (-z) might not be suitable for this specific job, as I understand, it’s meant to compress data during a file transfer intended over a network. In your commands current state, it’s just adding extra processing power which might not be useful for a connected device.

4: If you are transferring directories/folders, I found more consistent behaviour from rsync by adding a trailing slash at the end of a path. For example use “/home/username/folder_name/” and not “/home/username/folder_name”. I’ve run into recursion issues by not using a trailing slash.

Don’t use a trailing slash if you are transferring a single file. That distinction helps me to understand what I’m transferring too.

5: --delete will make sure your source folder and destination folder are a 1:1 match. Any files deleted in the source folder will be deleted in the destination folder. If you want to keep any and all added files in your destination folder, this option can be ignored.

--archive (-a) and --partial --progress (-P) are both good and don’t need to be changed or removed.

If you do happen to be running into a recursion issue that’s filling up your storage, you may need to look into using the --exclude option to exclude the problem folder.

Personally, I have no more tips that those that have already been presented in this comment section. What I would do now to find out what’s going on is the age-old divide-and-conquer debugging technique:

Using rsync or a file manager (yours is Dolphin), only copy a few top-level directories at a time to your external drive. Note the directories you are about to move before each transfer. After each transfer check if the sizes of the directories on your internal drive (roughly) match those on your external drive (They will probably differ a little bit). You can also use your file manager for that.

If all went fine for the first batch, proceed to the next until you find one where the sizes differ significantly. Then delete that offending batch from the external drive. Divide the offending batch into smaller batches (select fewer directories if you tried transferring multiple; or descend into a single directory and copy its subdirectories piecewise like you did before).

In the end you should have a single directory or file which you have identified as problematic. That can then be investigated further.

Maybe you have hard links or sparse files in your source directory. Try with -H for hard links first. You can try with --sparse but I think hard links are more likely.

Could it be you have lots of tiny files and/or a rather large-ish block size on your SSD?

You can check the block size with sudo blockdev --getbsz /dev/$THE_DEVICE.