Allocate space on create
By default, Azureus (since version 220.127.116.11) allocates all the space that a torrent will need to download as soon as the download begins.
To have it gradually grab space as it downloads: Tools > Options... > Files > Check "Enable incremental file creation"
The reasons for allocating the files at once:
- Attempting to avoid files getting fragmented on the disk
- Additionally, as pieces of files are downloaded in random order, the last piece might be downloaded rather soon, in any case.