ReadyNAS Ultra 6 doesn't want to backup to WD MyBook 14 TB

Hello.

First of all. I don't know much about network, computing or protocols. I'm just an average guy that need to get my backup working so please be nice to me even if I will not understand everything you say. I'll do my best. English is not my native language either so please excuse my errors.

I've a ten year old ReadyNas Ultra 6 that I've used as the primary storage space for my computers for the last decade. Today I've over 5,4 TB data on it, personal photos, videos, 20 year of journal entrys and so on. A few years back I had it connected to an external USB-drive that served as a backup for the NAS. However, three years ago the USB-Drive went full and since then I've not had any backup for my NAS. Risky and stupid I know.
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Today I bought a WD MyBook 14 TB drive to use as a new backup medium but I can't make it work with my old NAS. My NAS has three USB-ports, two USB 2.0 in the back and one USB 3.0 in the front. I've tried all of them without any luck.

When I connect the external drive to any of the ports the NAS display tells me that a USB-drive is connected so it seems to recognize it so that part seems to work. I've also connected the USB-drive directly to my Windows computer to see if it's okay and it works as it should. No errors there.

So, the USB-drive is recognzed and it's not broken. Still, I can't get the NAS backup to it.

I've created a backup job with the folder "backup" on my NAS as the source and the USB-port where I've connected the drive as the destination but whatever USB-port I use, Frontview tells me that the backup failed and that "the destination for the backup job is a usb-drive that's not connected at the moment. If it's connected, remove it and connect it again" (freely translated from my language). However, the drive is indeed connected and spinning.

I don't understand what's wrong. When I first connected my old USB-drive many years ago I just connected it, created a backup job and it worked perfectly. I've done exactly the same this time.

After looking around in Frontwiew I found the tab "volume settings". According to that the USB-drive is found as "USB_HDD_2" and the size is 13039 GB. However under the headline "FS" it says "No file system". Can that have something to do with my issue? The USB-Drive clearly has a file system as it works fine under Windows when it's directly connected to my computer but the NAS doesn't seems to agree.

The tab gives me an option to format my disk to FAT32, EXT3 or EXT4 but I don't want to do that in case I need to return my drive to the store. If I do such an action they will certanely not accept my return.

I thought that all external USB-drives were compatible with my NAS but maybe that's not the way it works. Have I accidentally bought a drive that can't be used by my NAS? That would be a very expensive mistake...

In another forum I read something about setting up RSync but that seem to be way over my knowledge and I wouldn't even dare to try in case I would do something wrong so that I lost all my files or made them unavailable.

In some way I need to get it to accept my USB-Drive (or in worse case another USB-Drive) because the NAS is ten years old after all and it can fail any moment.

Does anyone have an idea? I appreciate all help.