Yes, I know about RAID. I doubt the OP wants to go into RAID territory for using two 1 TB or four 500 GB HDD to get a striped RAID, just so one can have 300 or more MB/s write and read speeds, which aren't doing anything if the iMac has an HDD and not an SSD.Not true! Here are two examples, both RAID: 1) Lacie 2 Big 4 Tb for $599. Connection speeds of over 300 mbs. 2) Promise Pegasis RAID 4 Tb for $1,029 with faster connection speeds. Both units use 7200 hard drives. The more drives the faster the speed. If you are thinking about using one of these, make sure you maintain a backup.
yes he could use a firewire 800 as long as he is not looking to boot he won't need the thunderbolt .People also seem to forget that these blazing fast interfaces are also constrained by the source media too - if you're copying to you hyper-fast Pegasus TB raid from your internal iMac hard drive the bottleneck certainly isn't the TB or the external.
What do you need a 2TB external for? Thunderbolt might not be what you need.