mac mini m2 writing speed to external WD Elements Hard disk Is too slow

A quick calculation gives the transfer rate you speak of at about 10 MB/s. While the HDD you have would be a bit slow it really should not be that slow. However, are you really transferring a single file that is 12GB in size or is it a 12GB folder with many files inside? Transferring a lot of small files will always be slower than a single large file of equivalent size.

Which port are you connecting it to on the Mini? What cable are you using? It is possible that the cable is bad and not making proper connection.

How old is the drive? FWIW, I have found in the past that the WD enclosures have reliability issues even though the drive inside is perfectly fine. I have had many a WD external drive that performed poorly or out right failed long before it should and ripping it apart and putting the drive itself in another enclosure it would work just fine.

Your hard drive should be formatted to Mac OS Extended not NTFS.

well the hd is not a ssd but an old 5400rpm mechanical hd it would by no means sustain a transfer rate higher than around 80MB/sec to 100MB/sec

as appose to an ssd which would likely be limited by the usb3 interface