I've been using two extenal USB2 drives with a Linksys NSLU2 Network Storage Link for over two years now. The two drives, one a Samsung, the other a Seagate, have been on contunuously in cheap enclosures with no fan for the full two years. They both get mildly warm to the touch and I have had no problems.
I have had no problems with other USB2 drives plugging them in when already running, or plugging them in and then turning them on. Just be sure to unmount the disk before disconnecting when you are done.