Question about Samsung PM981a bulk

OK. Since you seem to have multiple M.2 ports, you'd have to tell us exactly what you have. Do you have a tower or a laptop? What kind of device? How are the M.2 connected to the motherboard? What kind of motherboard?

Am I interpreting this correctly: You have a tower with a mainboard and on one PCI-E interface of the mainboard there is a plug-in card with four M.2 connections. But I'm not so sure.

And, yes, what totally confuses me, apparently the SM951 is available as AHCI and as NVMe... I didn't know that.

Update to the original question:
In the meantime, I have managed to get the system running under Windows, but I still have specific symptoms. If only this SSD is installed in the system, then two data media are recognised. One with 0B, one with 512GB. As long as another SSD with 512GB is installed in the system, it looks basically the same, but the other data medium is effectively used.
Specifically in my case: SM951 is already in use as the system data medium, PM981a will be installed later as the data drive. Then only the 0B data carrier is recognised by the PM981a and the other "disappears" behind the signature of the system data carrier SM951.

As soon as and if I find a solution, I will make the results available here.

Many thanks to all who have helped with solutions!

In the end, everything worked. However, I first had to upgrade the bios (mine was really "old") and find out about the bios settings for double assignment of the respective interface (M2 slot was shared with SCSI interfaces 4 and 5) before it worked.

No. But a small note: M.2 is not the same as M.2. There are those with AHCI and those with NVMe protocol. The PM981a supports the faster NVMe protocol. The SM951, which was previously installed in the computer, supports the AHCI protocol. The two protocols are not compatible with each other.
Or to put it another way: the interface on your computer does not recognise the M.2.
Btw: you can even filter for the protocols at digitec ;)