To start, the LS Pro Duo (Raid 0/Raid 1) systems have actually been very disappointing from a performance perspective.
If you require a Buffalo product w/ 2 drive Raid 1 or Raid 0 that performs well, the Terastation Duo is a good pick, but yeah bit expensive :(.
Otherwise, if you're happy w/ single drive solutions like you currently have, the Linkstation line has what you're looking for:
http://www.buffalotech.com/comparison-charts/network-storage/linkstation/
The link shows model comparisons but to summarize what you're looking for:
1: Performance: Linkstation Pro LS-XHL performs the best of the linkstations.
2: Larger capacity: pretty much any of them.
3: WebAccess w/ uploads: All of them.
4: FTP Services: All of them.
5: Backup to USB: All of them.
Therefore for single drive Linkstations, the Pro LS-XHL is the best of the bunch.
Hope this helps you w/ the buffalo product line offerings. Remember: Linkstations are NAS devices (LAN connected) which should be what you're used to w/ your Linkstation 320.
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