Readynas Ultra 6 Disk Compatibility

Hello!
Proud owner of a 5-6 year old ReadyNas Ultra 6.

Firmware 4.2.28 

Latest RAIDar running.

So sadly over the course of the years, firstly my 5.th drive died - Due to financial position I was unable to replace at the time.

However the drive I was using was still compatible: WDC WD20EARX-00PASB0 1863 GB

Now, unfortunately today, I spotted that disk number 3 had gone "Dead" as well.

So I am rather panicking about my data, and wish to replace the dead drives.

Unfortunately this drive is End of Life, but.. Simple question really.

As it's Western Digital, 2 TB, what would happen if I put a SeaGate 2TB disk in here? (If it's on the compatibility list)

Would my dear NAS handle this and start to work with it, as if it was a Western Digital? Does the brand matter, or can I safely use any brand, as long as the Size matches and it's (preferably) on the compatibility list?

Any experiences out there? Would it go well?

There is still a few disks on the list that is not fully End of Life, and I would like to snatch them before they are discontinued too.

Any assistance and advice is appreciated.

Regards,