BTW:
http://www.ovh.co.uk/items/usb_disk.xml - that mentions the setup fee and "Professional" option. Nevertheless, if this isn't explained (again) at time of order, I do find that misleading - as in "rogue used-car dealer" misleading...
Anyhow, depending on the disk usage - or better said, utilization - you could look into the Kimsufi FS range (low end) or HG X...L range (top end) to use as an external storage solution. Then use something like NFS to access the storage from your existing servers - they can remain largely unaltered then, whilst existing customers can be moved onto the storage server(s).
Another alternative is GlusterFS, but when working with existing servers this will become a very cumbersome and demanding task. It would involve converting existing servers' storage space into "storage bricks", shared across all servers. So if one server had 10 GB free space, but another 100 GB, you'd have 110 GB to share between the two.
But both these alternative options would also "eat" at your bandwidth. So 100Mbps connections aren't recommended and if you were to guarantee a certain speed, this would be reduced then (due to the storage server traffic). Just throwing in some options though