I've got a bunch of usb external hard drives, but I don't trust them for storing digitised footage. I will have potentially up to 3 TB of footage after digitising. USB can be slow for transferring large files. Is there a faster alternative? and how do I know what I buy is reliable? E.g. is it better to have 3 1 TB disks or 1 3 TB disk? Is it better to go for an internal hard drive in a PC or an external disk drive?
Don't trust any hard drive to be reliable! Always backup your footage, you can not trust any HDD, doesn't matter how reputable the manufacturer is. There are always monday batches. HDDs WILL fail at some point, so ALWAYS make backups. Bear in mind that you might find a RAID solution nice to use but that's not a real backup, backup drives are disconnected and are not running with your "live" drive, otherwise you might end up with all your drives failing together, including your backups.
I recommend going for at least 2x3TB drives, they have a lot better price per GB than 1TB drives. Though 1TB drives tend to last a tad longer on average but thats not worth the much higher price in my opinion. If you want to invest more for higher product life, don't buy standard desktop drives but instead a workstation or server HDD. For Western Digital (WD) that would be the Red/Black/Purple series. Seagate has a few 24/7 Surveilance HDDs that are also more reliable.
In terms of speed. I'm guessing you are using USB 2.0. An alternative is USB3.0, the speed of that exceeds the read and write speeds of any HDD. If you are a Mac user you also have Thunderbolt in most newer products but external HDD cases for that are a hell lot more expensive than USB 3.0 cases but they have the advantage of daisy chaining, which means you can connect many external drives to one port without a hub if you need multiple drives connected at the same time.
In terms of what is better, external or internal. For HDDs that doesn't really matter anymore in terms of speed, as long as you get a case with a quality controller and dont use USB 2.0 or Firewire. It depends on what suits you better. A case is of course more portable but there are also hot swap cages for backups that are installed like DVD drives. Choose what you prefer.
And a general advice, never buy a complete solution where you buy a hard drive already enclosed in an external case. You do not know what you get and they are barely cheaper than getting your own case and installing the HDD yourself in a few seconds.
Personally, I use a 12 TB RAID array composed of 5 3TB drives. If any one of them fails, I don't lose any data, but if 2 or more fail at the same time, I am up a creak.
Other plans can consist of RAID 6, which allows for 2 drives to fail, but at the cost of two drives worth of storage capacity rather than one (so 5 3TB drives would only give you 9TB of storage rather than 12).
Alternately, some people prefer either RAID 1, where files are stored on two different drives in full or simply drive mirroring where files are kept on two drives manually and one is moved off site.
Historically, I used to use optical media, and I still occasionally use BD-R disks for some specific backup scenarios, however, in general, the expectation is that HDDs should last longer than burnable optical media (though both do degrade over time.)
It ultimately comes down to how much risk you are willing to live with and how much you can afford to spend on backups. Really complex setups can include things like dual RAID 6 arrays, one kept on site, the other kept off site.
Cloud archive storage is also an option where you pay to have your data massively replicated over diverse geographic regions so that no one disaster can wipe out your data, but this generally has a relatively high cost and has high bandwidth needs as well, rather than being a one time cost.
As for connections, you can use eSATA, USB 3.0 or Thunderbolt for quick connections. These will all keep up with the speed of most HDDs that you would want to use for archival purposes. Size wise, smaller drives are likely more stable as they utilize larger size blocks, but you still want to make sure you go for high quality drives that have a focus on data longevity. Most HDD manufacturers have drives intended for long term backups, though the amount of benefit you get from these drives is somewhat debated as quality of basic model HDDs has risen significantly in recent years compared to the low point.