karavite
Registered
Just my two cents, but I have been using 4 external drives and 3 internal drives for years and reached the terrabyte level of storage some time ago! 
I have worried about my externals wearing out, so I use them primarily as backup and for video files when running FCP. As a result these drives are only on and mounted when I need them. It is not at all a hassle to turn them on or off and/or unmount as needed. So, I say play it safe. Don't leave them running all the time. There is no need.
I have worried about my externals wearing out, so I use them primarily as backup and for video files when running FCP. As a result these drives are only on and mounted when I need them. It is not at all a hassle to turn them on or off and/or unmount as needed. So, I say play it safe. Don't leave them running all the time. There is no need.Kees Buijs
Registered
Just my two cents, but I have been using 4 external drives and 3 internal drives for years and reached the terrabyte level of storage some time ago!
There is no difference in the internal mechanicms for an internal drive and external one, so if you leave internal running, you can also leaving external running.
Ofcourse you can not turn of internal ones.
For security reasons and life span, hooking them up only when needed is not a bad idea, but if used on a regular base daily, leave them on all the time. Most drives die when turned on/off, so turning them on/off once is a lot better as turning them on/off multiple times a day, even if used during those times turn on for short times.
Drives not hooked up to a computer can not be damaged by that computer (lucky with mac that chance is VERY small), but operator errors could cause damage to the contents.
Good luck, Kees