Hi

Well, conditionally. External hard drives are not really intended for continuous operation. So I can't tell you how long the hard drive will last if it runs for 24 hours.

Stupid question, but are portable USB hard drives really more vulnerable than 3.5 inch drives installed in PCs?
If yes....why?

Yes. Of course it will break, but for the price of a quality server hard drive, you can get plenty of them here.

You could buy a normal hdd with an adapter (satapower and sata III to usb 3.0).