Time Machine share question

Hello all, I’m looking for some clarification/help.

I have the EX2 Ultra on a home network, with a mix of Mac and PC users. I have the EX2 setup in RAID 1 for drive redundancy. I have a variety of centrally stored documents there. (I realize that RAID 1 has a number of ways the data can get lost, but it’s better than nothing.)

However, it occurred to me that’s not an efficient use of storage for Time Machine backups; I already have the local copy of the data, the backup doesn’t need to be redundantly stored.

So I’ve been exploring using an external USB drive connected to the EX2 as a simple network drive for Time Machine backups.

However, when I try to select that share as a new Time Machine drive, it either does not appear in the list, or when it does and I select it, it keeps on asking me for username and password to access that network drive. I enter the same credentials as when I access it successfully in Finder, but it keep on popping back up in a never ending loop.

So I tried a different tactic, I went to the EX2 Settings/General/Time Machine and selected the external HDD as the share point to store Time Machine backups. After selecting OK, if I go back into that dialog, it hasn’t taken. So I assume an external HDD is not supported here.

I guess I’m not fulling understanding the requirements for Time Machine, and could use some help.