Short answer: you can't. Files (map data, firmware updates etc.) are always downloaded to %ProgramData%\Garmin\CoreService\Download (typically C:\ProgramData\Garmin\CoreService\Download)
The only solution I could find is to move that folder to another partition (make sure to completely exit Garmin Express first), then create a link to it:
c:>cd C:\ProgramData\Garmin\CoreService
c:\ProgramData\Garmin\CoreService>mklink /J Downloads "d:\Garmin.Downloads" Junction created for Downloads <<===>> d:\Garmin.Downloads
I've been using the staticmaplite library for a while to generate static maps based on OSM tiles.
Recently I started getting the following error when downloading tiles:
Access control configuration prevents your request from being allowed at this time. Please contact your service provider if you feel this is incorrect.
Based on that info, I checked how staticmap.php was fetching tiles and noticed that the User-Agent string was hardcoded to "Mozilla/4.0". I fixed this and now I can fetch tiles again. You can get the updated version from here: https://github.com/crox-net/staticmaplite.