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It should be safe. You might have to do a little troubleshooting for the first boot with the new drive.

The risks that I can see are:

* When your motherboard detects that a new drive is added, it might change the boot order, and attempt to boot from a drive that doesn't have an OS. 
  * To fix, just go into BIOS setup and fix the boot order.
* Commercial OSes like Windows may decide that your hardware signature changed and you now have to buy a new license. 
  * I would search online first to see if other people have encountered this problem. With this type of DRM it's hard to predict exactly what it will do.
  * To fix, you would probably need to check with their customer or technical support and say you're planning to add a drive.
* With Linux, if you use the deprecated approach of identifying drives by a string like `/dev/sda` rather than UUID, those might get changed with the new drive resulting in Linux crashing at boot. 
  * Virtually all distros default to UUID, so this shouldn't affect you unless you have a very old or very badly configured setup.
  * To fix, you can switch `/etc/fstab` etc. to UUIDs before adding drives.
* If you have some unusual configuration (you mention RAID) it may or may not break when you move drives around. It depends on how exactly you set it up. 
  * To fix, you would probably have to check the manual (and your notes) on it.

The theme here is that it may make your computer unbootable, but usually this is easily fixed (but may require spending money). Do have a secondary computer available so you can go on the internet for troubleshooting. A bootable USB drive is also a good idea.

I don't see how it would normally erase or brick the drive. When a normal system encounters unexpected drives, it should ignore them rather than defacing the data for no reason. For example, RAID shouldn't just blindly take over a new drive like a cancer without asking you, but I'm not familiar enough with RAID to speak for every RAID driver out there. Also, technically I can't know for sure that you don't have some script running at boot that says "if new drive detected, nuke all data".

If you want to be paranoid, you could avoid moving existing drives, and only plug in previously free cables to the new one, while also preferring higher-numbered ports (eg. `SATA_05` before `SATA_00`). Usually when software is automatically arranging the drive IDs, it goes in the order of port codes, so if adding a drive changes anything it is more likely to affect higher-numbered ports only (depends on exactly how the software works). But this will only save you from the easy to fix drive order issue, it won't save you from the (very hypothetical) "insane, malicious, rogue RAID" scenario above.