That is the only USB drive.
Specs:
M/B: MSI - Z97-G45 GAMING
CPU: i7-4790K
HVM: Enabled
IOMMU: Enabled
Memory: 32 GB (max. installable capacity 32 GB)
Network: bond0: adaptive load balancing, mtu 4000
eth0: 1000 Mb/s, full duplex, mtu 4000
eth1: 1000 Mb/s, full duplex, mtu 4000
Array consists of 4TB, 3TB, 2TB, 1TB spinners. 500GB, 2x 250GB SSD's All connect to m/b or PCI-E SATA card.
The only thing I can think of is move all data from 4TB array disk to new disk then use the array disk as parity instead. I'm reluctant to do this due to the wear on the drive and downtime etc... but if the sizes appear to be the same, what's stopping the current array drive being too small for parity.