I work with W10 and try to use SIRIL 0.99.4
Most of of my data files are located on extractible HDD without letter but accessed as a link thru a directory.
Such directories seem not listed in SIRIL “openfile” interface. How to work around
You should mount your external drive so it has a drive letter.
Siril probably doesn’t support these windows-specific links which are not real files. If this is for processing, you should really not do it through an external drive because of the speed, and avoid doing it on a rotating disk at all to limit its wear.
You should also update your siril version because 0.99.4 is about 4 versions old.