how to access external devices?

I have an external HD connected to my home network. I prepare documents in writer for example, but I can not find the way to save this to my external HD. Can you help me, please?

This question in fact is about “How do I use my external device”, which isn’t a LibreOffice specific question at all. And if you don’t specify any technical details (Typ of external device, NAS (network (?!?) device, USB device, operating system + version) nobody, even if asking in a forum appropriate for this type of questions, could provide any helpful answer.
(making your device available to store files to the device is a matter of integrating the storage device into your operating system in order you just could do a File -> Save As within any application (incl. but not at all limited to LibreOffice).

…having just mentioned FreeFileSync in another question I think it is relevant here also>

FFS(:O) (or another similar software) set-up correctly with command-line and batch-processing, filters, and synchronize options can solve this for you: e.g.

  • set up batch-job to create auto-copy in dedicated local folder
    -set up second batch job to copy the chosen files to external drive when that drive is detected by FFS (running in background, and auto-started with operating system)

Good Luck!