I think the old trick to create a secret partition and then place malware in there still works and will never be fixed because the AV has no access to other partitions, which make AV detection more or less easy to bypass. You however need physical access or - and admin rights to create the partition.

There are some cases, which were discovered some years ago, by Kaspersky from undeletedable malware, which basically did that, except that the HDDs at that time came with those secret partition preinstalled, that contained a modified firmware. I think that was to infiltrate state level based operations.