While leaving the HDD powered on likely won't harm the drive in any way, given the Malware/Ransomware threats of today, it's best to detach externals once a backup is completed or data transferred. Because once such an attack starts, it's like a snake crawling in the grass until all of one's files are encrypted & then when one gets the notice to pay a ransom to obtain one's backups/other files, any drive images on the external becomes useless.
Up until a couple of years back, I kept an internal drive on every PC for the purpose of backup alone, and still run a quickie internally before Windows update for just in case an update messes up the OS. Although I create my regular backups on externals that I assembled myself, mainly from systems that shipped with a 1TB HDD & upgraded to SSD, these are detached ASAP after backup & powered down.
So even though the My Cloud backup drive may be designed for 24/7 usage, doesn't mean it's a good idea to leave plugged in & powered on. In fact, other than the internals that I formerly used for all, have never left a external plugged in 24/7, other than the risk mentioned above, not all of the cases are equal, those with a plastic outer layer will run warmer than one in an aluminum enclosure. And for every degree measured in C increasing, this often leads to a shorter lifespan. unlike the models of the past where we could carefully pry the casing off & installing the drive into an aluminum model (sometimes with a fan) or use as a desktop drive, it's not possible with most newer models.
This is because the former SATA connections are removed & the only one is where the plug goes in. While possible & only under warranty one may luck up & find a most aluminum case, that's rare, because these type of drives hasn't been on the market long enough to create demand. Plus most are still under warranty, if not, the model isn't worth purchasing unless priced very low (read reviews very carefully), once that input goes bad, these goes all of the data stored, the drive is useless.
So be sure to only unplug (when possible) from the computer end of the cable & leave the one to the drive in place, to prevent accelerated wear in the area. This is likely why there's a high amount of refurbs of these externals, the OEM likely had to exchange & replace the plug, which brings up a third & final issue. The data stored is no longer private once sent in for RMA, the employees who repairs the drive are going to test it & will have access to any stored data, unless encrypted.
Therefore, due to these issues, especially the first situation mentioned, it's best to properly eject & detach the cable after any type of data transfer, or pay the price if left plugged in & powered on.