[ZrByte]

Just to echo what others have said it will be fine. I used to have a twin bay external usb/esata drive enclosure that ran 24/7 for 3 years before I found usb 2.0 wasn't quick enough for me so I put them internal instead. In my case it was a DIY enclosure so I just used old desktop drives in it but you will find any off-the-shelf external hard drive has a normal internal hard drive at its core. These drives are ofcourse fine for 24/7 use and the only factors could be how well designed the enclosure cooling is or if your drive is in direct sunlight, as heat can have a dramatic effect on the speed and lifespan of a mechanical hard drive.