My first recommendation is don't. They usually throw green drives in them and they seem to fail more quickly than others as surveillance drives. My second thought is- they come on sale bloody cheap compared to component drives, so if they're half price and short term budget is a serious concern well why not use one as secondary edge storage? That doesn't replace your primary storage needs though. Just be in the habit of checking every day to see if it's still alive. If it dies under warranty, replace it quickly. I've used an external USB drive plugged into my router as 24/7 video stream NAS edge storage for cameras before. Someone gave me a Walmart gift card so it was more or less free. The safest bet though is to get a WD Purple or Red drive and have it installed inside your desktop PC. You'd hate to have the cheap drive external go down when you're depending solely on it. Also, if you use the external drive for motion storage instead of 24/7, you might miss events because the motion event can't be written while the drive is asleep or waking up which will happen often, particularly overnight. There's no advantage to using a full price USB drive instead of a component drive inside your PC.
Sooo, basically don't do it unless it's as a cheap backup for 24/7 recording. Otherwise, there are too many possibilities for failure or missing footage even if it doesn't die. Stick a proper drive in the PC first. Then, if it's bloody cheap or free and you want some redundancy, use a USB drive. Best bet though is a proper NAS (Google Synology) as backup edge storage with a red or purple drive in it in addition to one in the recording PC. Hide the NAS in a closet or in a dropped ceiling separate from the PC so if you get burgled they won't find the NAS. 24/7 recording works great for that because once they steal your PC there's no more footage to overwrite and you've got a perfect recording of everything in case your motion alerts missed something earlier. Just tie off the ethernet cable a few times during the run so if they yank on it from the PC end they don't find or damage the NAS unless they trace the wire the whole way. Your alarm system should be ringing though so they shouldn't have much time before the cops get there.
