Can a power loss break a hard disk?

Power loss CAN damage a harddisk, but it would be unlikely, yet if you feel uncomfortable or just want to be a 100% sure, consider buying yourself a fine brand new hard drive... The fact that your harddrive is giving you I/O messages isn't a good sign anyway...

We've found that in many cases it isn't the power loss that causes the problem, it's the power surge that happens when power comes back that can cause problems. Not all power strips are created equal in this regard.

Power loss shouldn't do any physical damage to the disk, as most modern disks use the inertia of the platters to safely park the head when they suddenly lose power. However, depending on why the power went out (power surge tripping breakers, for instance), it's possible damage was done that way, or if the PSU in the machine reacted badly to losing power.