I've had trouble mounting SMB/CIFS External Storage recently in OwnCloud (currently version 9.1). External storage mounts have always been flaky in OwnCloud, but this time I think I found a solid solution.
While troubleshooting this problem, I ran across an issue posted on GitHub where the suggestion was made to try connecting to the Samba shares from the command line on the server using smbclient. This would at least prove that the Samba shares are accessible, and confirm that you're using the right name(s) for them in the OwnCloud Admin interface.
From the command line, I ran
smbclient -L localhost, which confirmed for me the names of my Samba shares were correct. I also noticed, though, that it listed a Samba DOMAIN, which I had forgotten about completely. Entering that Domain name in the appropriate field in the OwnCloud admin/settings set everything right. So, now I have Samba shares that appear as top level folders in OwnCloud for multiple users.