Just throwing this up here for anyone in the future who has the same proglem I did and is pulling out their hair trying to find a solution.
I'm on Windows 10 and I have a fresh install of OMV 7. I was trying to log into an SMB share I had created and it wouldn't work no matter what I tried. Everything I found online just explained the steps I had already done and made it seem like it just works. Now I've set up SMB shares in Windows and even in Linux, admittedly the latter was well over a decade ago so I don't remember much about how smb works, so I somewhat knew what I was doing.
Anyway. I ssh'd in and looked for a log file. After checking /etc/samba/smb.conf I saw it pointed to /var/log/samba/ as the log directory. I had to turn on logging, which can be done through the GUI in SMB/CIFS settings or by editing smb.conf and restarting the service, though be aware the latter will be overwritten the next time OMV updates the SMB settings. With logging turned I was able to see that even though I had created users, and given them read/write access to the share, smb wasn't recognizing the users. I kept seeing check_ntlm_password: Checking password for unmapped user. After trying a few things in vain I did a little googling and found a post that said they solved this by adding ntlm auth = yes to smb.conf. I tried it, and it worked! I then went back and added it in the GUI under extra options at the bottom of the settings page for SMB/CIFS so it would persist.
Why this worked I don't know. From what I understand I didn't want it using ntlm, so why did adding ntlm auth = yes do the trick? Perhaps someone more knowledgeable than me can pipe in with an explanation.
TL;DR If you're having trouble logging into a SMB share, turn on/check your logs and check if you see something along the lines of check_ntlm_password: Checking password for unmapped user. If you do, add ntlm auth = yes to the extra options section on the SMB/CIFS settings page.
