So, what kind of external had drives can be used for editing? I'm talking of 1920x1080 material, mostly.
I'v e always used external USB drives just for back up, and since I use PC I don't plan to have Thunderbolt soon.
What kinf of external HDs can be used for editing on them?
Bare minimum: firewire 400. The best: Multi bay Mini SAS enclosure, drives in a RAID 5. 7200 rpm drives. I've had good luck with Samsung and slightly less good luck with Hitachi's enterprise drives.
I use external usb drives with no problems, just be sure they are 7200rpm. With my i7 and Vegas Pro 12 I have never had any issues using various brands of drives and currently have 6 connected to my computer, and a couple more that I swap in and out. Recently I have been capturing my 1920x1080 Intra100 P2 files to a network drive and have been editing projects in Vegas with the footage on the network drive, both video and audio, and have had no problems. Normal projects are 2 cameras and 6 - 12 multi-track audio files.
I'd say just try a drive out and see how it works for you. From my experence I'd say you should be fine.