I'd like to make backups on an internal hard drive (using an USB enclosure) and carry it off-site. Will the hard disk survive for a long time?
How long is a piece of string? It all depends how carefully you treat the drive.
Internal and external hard drives are identical - you'll have exactly the same chance of survival with an internal drive fitted into an enclosure as you'd expect from a ready-made external drive except perhaps for some purpose-built drives such as the LaCie Rugged All-Terrain Hard Drive:
For maximum safety, use a solid state disk - these babies can take some serious abuse. Of course the robustness comes at a price.
I have moved around sub-300MB Seagate hard-disks (when they were bleeding technology) in my jeans pocket to carry data around... they did not last centuries, but worked well in their time.
Yet, I would discourage such escapades in general.
Consider anti-static covers and hard casing to transport disks in general.
Consider also a portable drive (Seagate, Iomega, and many more make these).
Or, better still, move to solid-state technology
(you get 32-64 GB flash devices in various formats these days).
And, larger and more robust technologies are on the horizon.
Definitely. You can use an external enclosure or even those static-proof bags that hard drives and other hardware come shipped in.
Always treat it with care. Keep it away from magnets, heat, extreme cool, etc and you should be fine for a long time. My company does this with 3.5" external drives, we transport them frequently.