It all depend on what your difinition of safe.
I have done it a few times with IDE device, because I was press for time when worked on a small company lame old sever. The chep boss had Win2k DC, File, Exchange 5.5, II & Norton Antivirus Server on a P133mhz & 64megs of ram.
My old boss give me chest pain once, when he pull the power plug on a brand new server that shut down due to the detection device that that 30 sec to reset it self before we could power it up. It sould be fine to remove the device while the power is on if the mobo doesn?t support voltage disruption.
The only problem is the OS & BIOS still see the device as mounted, and crashes when that device is query (only have done it with MS-DOS, NT4, Win2k & Linux.) Luckily Linux/Unix you can mount & un-mount devices before you commend the action. I don?t think it matter if you unplug the power then ribbon cable or ribbon then power, but I tend to unplug the power first.
I don?t know what would happen to a computer system if the power pins were accidentally reverse when it is remove, but could only imagine it wouldn?t be a pretty sight. But, the chance that the power happens to reverse during a removal process is highly unlikely.