Hello,
Whart products are you trying to download content for? And is this one job trying to download everything all at once?
If so, that could be your problem there. For example: SEP 11 and 12, client and server, V2 and non-V2, Mac and non-Mac, hub and non-hub.... if the temp location, permanent stoarge location and distribution center are all on one disk, that could very well total up to more space than the LUA server can handle in one go.
Secondly, I believe moving the clu-prod and creating a junction is not supported.
Here's an article many have found helpful:
Managing LiveUpdate Administrator 2.x Space Usage
https://www-secure.symantec.com/connect/articles/managing-liveupdate-administrator-2x-space-usage
Try a series of smaller jobs and it should work fine. Also, have a look at what you need and what LUA is actually configured to go get--- there are usually some tuning steps that can cut the burden way down.
Hope that helps!!