First, I am not a Drivepool pro and this is just what I would do if I were in your position and cannot guarantee your data. First, is it the number of drive that can fit into the case or is it the number of SATA port you have available. If it is the number of SATA ports you could try getting a SATA card. You could also use the USB option you said you had. Attach a new 4TB drive by USB, add it to the pool, then remove one of the 3TB disks. Drivepool should move your files to the attached USB 4TB disk when you remove the 3TB disk in drive pool using the remove disk option(Will take a long time over USB). Then add another 4TB drive where the 3TB disk was once the files are moved off of it, add to the pool and remove another 3TB disk and repeat. When you have removed the last 3TB drive add the 4 TB disk that you have attached to USB into your server. If it is not automatically added back to the pool do it manually and your files should be all back. If anyone else here can verify that my suggestion would indeed work(or not) please speak up.
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Or, since drashna beat me by 2 minute, follow his suggestion lol