Network Password prompt when accessing private folder on NAS

I am setting up a NAS on my home network. The unit is a Seagate STCR4000101. The primary purpose of this unit is to store backups from the 4 computers on the network.
During the installation/setup, I was asked if I wanted to have a private folder. It was highly recommended that backups be placed in a private folder for security purposes. So, I initially set up one.
This unit is recognized on my network and I have access to all the public folders through Windows file explorer. However when trying to view contents of the private folder, I get a dialog box titled "Windows Security", prompting for a user name and password. I've tried just about every user name and password that I know that might be associated with windows or windows networking - none have worked. The only thing that I haven't tried is a user name and password for signing into windows. I don't have one because I don't use one. Is it necessary to have a user account that has a password and that password is the one that is being prompted for?
If that might not be the reason, what password/username is it requesting, and where can I find it if I have created one and I forgot.
I understand there is a way to get rid of the password prompt, but will that give me access to the private folder?
My network is through an ASUS RT-N66R router using wired connections. The network is comprised of 4 Win 7 computers. The computer I'm using to set things up has a Win 7 Professional O/S; several of the other computers are Win 7 Home.
I'm networking through a Workgroup (not Homegroup), and the workgroup name is not the default "workgroup". Somewhere in my searching, I think that I saw an indication that the NAS settings identified the workgroup name as workgroup. I can't find a way to get back and verify that however. The documentation on that unit seems to be a little thin.
Any suggestions/assistance on this would be appreciated. If I can't find a solution, then I'll probably just put the backups in a public (sub) folder. I haven't figured out yet how to create another public folder if it can be done.

You must of set a password when you set it up it doesn't know anything about Windows password it what you set on the nas for users

Thanks for quick reply.
I suspected that might be the answer. Now I have to find out what I might have set it to. I really love "sketchy" documentation. Maybe setting another private folder and remembering the password will help. I can always delete the additional private folder but can't seem to delete the initial one.
I'm still questioning why the dialog box was titled "Windows Security.

It's Windows security as it's that asking for the password via the nas

It worked!
I must have been sleeping. I found in notes I made when installing the NAS, a username and password and it worked.
Tthis was quickly solved. Thanks