Hey there again, @perblue!
Transferring data back and forth from/to a mechanical hard drive isn't dangerous or risky for the lifespan of the drives. However, keep multiple backups is highly essential, so it's better to copy them. Always remember that "backup" means that you have your data stored in at least two (2) locations. Moving data from your system drive to an external hard drive or another internal one is not a backup, unless there is already a duplicate of the file on a different drive!
The WD AV-GP hard drives are designed for high temperature always-on streaming digital audio/video environments such as PVRs, DVRs and IPTV systems. They, however, are not really suited for desktop usage. They incorporate features like low power consumption, IntelliSeek and IntelliPower that might make them appear rather slow in the PC environment.
If you plan on using RAID configurations and you desire 24/7 reliability in the HDDs in your desktop PC, then you should probably check the
WD Red HDDs. If you simply want to use the secondary HDD for massive storage, then a massive
WD Blue should be suited for the job as well.
Hope this was helpful. Let me know if you have more questions!
SuperSoph_WD