There are many types of hard drives out there, and what follows is a brief overview of what's available and what we prefer when you supply a drive.
First, let's get one thing out of the way: USB2 is almost never a good choice when you're dealing with large files. It's too slow - in reality it's usually far slower than the nominal speed of 480mbps (35MB/s). Please avoid drives that only have USB2 interfaces!
If we're scanning your film, we prefer at minimum a USB3 or eSATA drive. 2k film scans take up a lot of space, which means slow transfer times. With DPX image sequences, it's even slower because there's transactional data that has to move between the computer and drive for each frame, since each one is a separate file. For that reason, you generally want the fastest drive interface you can get: USB3 or eSATA. You can format the drive for Mac HFS+ or Windows NTFS.
File delivery to Gamma Ray Digital
If you're supplying us with files for mastering, authoring or encoding, the same basic rules above apply, but we can also work with Firewire drives - most incoming video goes immediately onto one of our edit systems, which are Mac-based. So, USB3, Firewire (800 preferred) or eSATA drives, in either Mac HFS+ or Windows NTFS format.
Note that at this time we don't accept Thunderbolt-only drives, but if your drive has Thunderbolt and some other interface on it, we'll work with that.
What to Buy
If you're purchasing a drive for transfer from our facility to yours, we don't recommend spending extra money on faster drive mechanisms. In reality, 7200RPM drives aren't going to buy you much of a speed boost over much less expensive 5400RPM drives, because the limiting factor is usually the interface (especially if it's some flavor of USB).
We get asked a lot which drives we prefer, and the answer is usually "the cheapest name-brand drive you can get." While that may not be a very satisfying answer, it's the truth. There are so few manufacturers of the actual hard drives that they're essentially all the same. What you're paying for is the look of the enclosure and the interfaces on the back of the box. So stick with name brands like Western Digital, Seagate, Hitachi, LaCie, GRaid, etc.
If you're shipping your drive, or will have us ship it to you, it's worth getting a smaller "pocket drive." These drives use more robust 2.5" drive mechanisms like those found in laptops. They're not only smaller and lighter (which means less expensive shipping), but they're more robust because they're designed to be banged around more.
Amazon.com has quite a few drives that fit the bill.