Correct Read/Write permissions on NAS (Newbie)

Dear OMV users,

I am currently running a BananaPi with armbian and OMV, which I want to use as NAS.

There are two devices attached to it:
- USB Stick
- External HDD (via USB)

I managed to set up the smb share and want to access it via my linux desktop computer. Unfortunately, I cannot access all the files and directories on the External HDD. On the USB-Stick, however, everything works fine. Since I am no expert in linux rights management, I hope some experienced users can give me some feedback on my setup.

Checking the smbshare on my NAS-System:

Source Code
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• ls -l /srv
• drwxrwxrwx 1 root root 4096 May 10 22:29 dev-disk-by-id-usb-General_USB_Flash_Disk_01070000000000E4-0-0-part1
• drwxrwsrwx 13 MyUser2 users 4096 May 10 23:25 dev-disk-by-label-NAS

So I think the problem lies here. My external HDD belongs to MyUser2, whereas the USB Stick belongs to root. I created both shares using the webgui, so I dont see my mistake here.

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NAS-System: ARMBIAN 5.25 stable Debian GNU/Linux 8 (jessie) 4.9.7-sunxi

root
MyUser1 (do I still need a dedicated user here? If I want to make updates to debian vie ssh I could use root for that. Otherwise I leave the system as it is anyway)

OMV 3.0.74 (running on NAS-System):
admin (for weblogin only)
ShareUser1 (created via webgui, the only user allowed to access the NAS share)

Desktop-Computer:
root
MyUser2

You were right! Some folders on the external HDD do only have access for myUser2 (=the user of my desktop computer):

myUser2 (group=myUser2)

drwx------  8 myUser2 myUser2      4096 Sep 18  2016 TestFolder

This makes sense, because I moved the files and folders onto my external HDD using my Desktop computer via USB. So after doing

chmod -R ugo+rw ./

I should be able to access all folders and files on my NAS via samba. Files on my NAS are accessed via samba user ShareUser1 (group: users) and this would be allowed by the o+rw option in the permissions.

The only problem is that, when I move a file via samba onto the NAS, it is then owned by ShareUser1 (group: users). In other words, when I plug my external HDD into my desktop computer via USB some time, I won't be able to read those files because I would access it with myUser2 (group=myUser2).

So either I change all files to o+rw (this makes me a bit nervous from a security point of view) or I apply the same group for myUser2 and ShareUser1 and allow g+rw. Would that work even if the users are defined on different systems?