"WD, Touro, Seagate external support sleep mode?"
No idea coz I don't have any of these models
"if left never unplugged from PC or NAS, no sleep mode over the time could kill the HD much faster .........?"
Can't tell. However, common sense telling me, something not designed for non-stop operation, and we stress it to run at 7x24, might lead to premature failure.
I've two transcend 2tb and one adata 2tb. All are permanently attached to (never unplugged from) NAS/Magic TV/PC. As the adata does not support sleep mode, it cannot (should not) be attached to NAS/Magic TV. Otherwise it will non-stop 'working' until the end of the world!
As my PC would power up for more than 10 hrs per day, the adata would then be non-stop 'working' for similar time. Without unplugging the hdd, there is a work-around by ejecting the usb device in Win environment. As such, it goes offline until next Win startup (not equivalent to sleep mode, just better than none!).
It seems transcend portable hdd is my real cup of tea!