okay here is a two cent review / how to from me (been there, done that)
1. first off, flash the 520GU to tomato ND Teddy Bear mod. it cant get any better or simpler.
2. then ensure that the usb drive you are attaching SHOULD be linux FS. even though the tomato mod supports ntfs, i'd suggest everyone to stick with ext3. it needs to have three partitions. first about 1gb for where optware will be installed. second as swap about 512mb and remaining as ext3 format.
3. download putty for use with windows. and start with the optware script as posted in the links above
4. configure mounting of disks at startup
5. download and configure transmission daemon. ensure to never download more than 2 torrents in one go and also reduce the number of peers per torrent to be around 30.
hopefully u'd be good to go. incase of any further assistance feel free to google or buzz us here !
review:
1. its a great setup provided you dont end up wanting too much from it. respect that ultimately its just a router with very very limited resources.
2. nas speeds are in the range of 1.5 - 2.0 mb/s when using wired, so dont fret for they being slow. even the better dedicated devices that are primarily nas like qnap and synology, just about double these speeds and no better for 7 times the price and more.
3. additional advantage of using and setting a ftp server in a snap.
4. further setting up a dynamic dns service to ensure u can access the transmission daemon from anywhere on the internet. goggle and u'd know how.
5. lowest energy consumption imaginable.
6. only thing seeming difficult is to get it going, post that its just a very very smooth drive.