Exactly something with links happened that had gone dead…
The HLDS links that you originally posted still work OK, but only if one initiates the download from inside the HLDS website. Trying to access the download links directly from the outside won’t work. It is probably a referer limitation.
If you go via this entry page: http://hlds.co.kr/v2/search.php?bo_table=product, select the drive from the list and then try to download the firmware from there, then everything works just fine.
That said, there are still quite a number of firmwares (those for GUD0N, GTC0N, etc, for example) which have invalid links and give an error, but all those mentioned in the list that I have provided earlier will download without any problem.