This is a mass storage device using USB for its connection. It *must* follow standards, which are OS agnostic. The vendor may not have ever heard of anything but "Windows" and "Mac" but that should not preclude the product's adherence to standards and therefore its use.
See:
https://en.wikipedia.org/wiki/USB_ma...e_device_class and
https://en.wikipedia.org/wiki/USB_Attached_SCSI for information on the technical standards the disk should adhere to.
I don't know what the 24-month warranty includes, nor what can be implied under the regulations of your country. I don't know EU law, but under directive 1999/44/EC the customer's country's laws and regulations -- if EU -- apply to the transaction, as the seller was in the EU.
As for OpenBSD -- if the drive connects to it
-- you can provision an MBR or a GPT. If the sector size is 512 bytes, an MBR will only address 2TB, though the disklabel could still address the entire drive. An MBR or GPT would be protective in nature, as those are recognized by other OSes.