Rsync to compare two Synology - NAS machines?

[Broodjegehaktmetmayo]

G'day all

After having been absent for quite some time (bad, bad, bad accident, I now have a subscription to six different hospitals; each makes sure it invoices a part, none knows the solution to my problems), I have started to sit behind my computer again (difficult, I am dizzy all day and I can't seem to concentrate).

To create order in the chaos of my 47 external disks, I have bought two Synology NAS-machines. One is the main NAS, the other one is the mirror/backup for the first NAS. I have put in disks in RAID6, and have copied some 6 TB of data to 1, and then, using that first Synology and its built in tools, have had that copied to the second Syno (took some 40 hrs). I've noticed a difference, around 40GB of data between the two machines. Synology unfortunately does not provide good tools to check this kind of situations, so I have to dive 'under the hood' to use rsync (Synology is linux-based). I'm afraid my lack of concentration hinders me to find out what command I should give.

Then I thought of you all nice and helpfull people, and I'd figured I come in here to ask if somebody could tell me how to do this. As I am still a noob, and many of you are so extremely knowledgeable, I thought this might work

I do not want to use rsync to do the syncing (I leave that to the Synology software as this keeps records of what is being done), but I would like to simple use rsync to compare the two machines, and tell me where the differences are. According to some, rsync should do this.

Both machines are on my internal LAN, they can see eachother, and I can enable telnet and SSH to them.

Would anybody perhaps be willing to help me out?

Thank you in advance for your help,

Bye