I am suspecting my PC to be running low on power, when I try to use an external hard drive (getting random I/O errors after I connect it, not all the time, only when both drives are busy). Is there any easy way to check if the power supply is supplying enough power for all components?
USB can only provide 2.5W per port. I doubt 5W is going to change your PC from being stable to random errors - if it was running that close to the edge of the PSU you'd be getting more issues. Sounds more like a dodgy caddy/controller on the external drive.
Try either testing the drive internally (take it out the box, plug it in with IDE/SATA); or a different external disk.
You fill in all the parts your computer has and it will calculate the recommended amount of Watts.
Generically, try using a killawatt plug http://www.thinkgeek.com/gadgets/travelpower/7657/
Worth their weight in gold over time.