`git annex sync --content` behaves differently for directory or SSH resources.

I use the directory resource on two external hard drives to sync between them. They always get mounted to the same mount point.

When syncing from one USB drive to another, git-annex iterates over all files in the annex. This takes a long time even if only one file got added.

When running git-annex in one of the USB drives and syncing with SSH, git-annex only does the file transfer necessary and does not iterate over all files.

I'd like to have the SSH behavior also for syncs between USB drives. Is that possible?

Also, how does git-annex make the decision?

UPDATE: adding the output of git-annex as requested by Joey

```
% git annex sync --content usb-sg2
commit                                                                                                                                                                           
Auto packing the repository in background for optimum performance.
See "git help gc" for manual housekeeping.
[master 3bREDACTED4b] git-annex in usb-sg1
 2619 files changed, 2619 insertions(+)
 create mode 120000 import/REDACTED
 ...REDACTED...
 create mode 120000 import/REDACTED
ok
pull usb-sg2
From /media/thk/thk-sg1/annex-sg1
   7dREDACTED45..caREDACTEDfb  git-annex  -> usb-sg2/git-annex                                                                                                                
   84REDACTED33..3bREDACTED4b  master     -> usb-sg2/master                                                                                                                   
Auto packing the repository in background for optimum performance.                                                                                                            
See "git help gc" for manual housekeeping.                                                                                                                                    
ok                                                                                                                                                                            
copy import/REDACTING ok
[FROM here, the above line repeats for all files in the annex]
```
