Unknown computer called ATP on my network

You can log into your router and learn about the IP and MAC address of the "mystery" device, at least.

You could also search the MAC address for the manufacturer (I am not sure how useful is that for you). Note that it could be faked.

Using its IP you could try accessing the device, depending on what it is. If it is a hard drive, you may try mounting its partitions. If it is a computer, you could try sshing to it, xrdping. That very much depends on your case. A lot of information is missing. Please make an effort and provide more info.

What do you see in the webpage for the devices connected to your router?

Are you certain ATP is your disk?

Do you have physical access to the disk?

How did you format it? (MBR/GPT, partition types, EFI boot, etc.)