Solution
My HDD is formatted as exFat which doen'nt seem to support the readlink operation.
I am trying to set up a gatsby page via the gatsby CLI. The 'gatsby new' wokrs and set's up everything nicely, as far as I can tell. When it comes to starting the develop server I get the Error like in the title: "Error: EISDIR: illegal operation on a directory, readlink './path/on/my/hdd/.cache' I learned, that "EISDIR means that the target of the operation is a directory in reality but that the expected filetype of the target is something other than a directory." (Using Node.js I get, "Error: EISDIR, read"). What drives me mad is, that a file called .cache wouldn't make much sense, so somewhere deep down in this gatsby jungle has to be something, that gets messed up.
I've tryed node Versions 10.16.0 and 12.2.0 managed via nvm on windows 10. I've tryed different Folders and Harddrives, I've force cleared my node cache, I've tryed different starter packages, I've tryed npm update on the node_modules installed via gatsby.
PS > gatsby develop
success open and validate gatsby-configs - 0.075 s
success load plugins - 9.615 s
success onPreInit - 0.003 s
success initialize cache - 0.074 s
ERROR
Unable to copy site files to .cache EISDIR: illegal operation on a directory, readlink
'C:\my-gatsby-website\.cache'
