I don't have an Asustor, but I use the hard drives in my own server under TrueNAS without any problems. They are 3.5" disks with SATA interface. So they should be compatible with Asustor AS6510T.

I had ordered one and had to send it back because only just under 1 TB of it was usable. Then they sent me a replacement. And the same problem with this one. So I sent it back again and didn't want a third HD. I now have my two old 6 TB disks in use again. At least they work.