I have two MyBook Thunderbolts and they work fine for mass storage of HD video files. They each went for about $600 for the 6TB editions. I've had them for well over a year now and am pretty satisfied with them. When the drives wear out, I will just go buy some new WD Green drives and keep on using the MyBook Thunderbolt enclosures for many many years.
You may also want to take a look at the MyPassport Pro. 4TB is $429, so just a hair above your budget.
On the Seagate side, I also have two BackupPlus Thunderbolt Adapter. I bought each for $190 a couple years ago, and then I bought 3TB BackupPlus hard drives from Costco and ripped off the USB 3.0 adapter and plugged them into the thunderbolt adapter. Works fine. Another bonus of these thunderbolt adapters is they are also good for bare drives, so whenever I need to quickly get data off of a bare drive, I just stick it onto the Thunderbolt Adapter and data gets transferred FAST!
Doing SSD to SSD thunderbolt transfers, I managed about 700GB in 20 minutes. Not bad I'd say.