Re: my fusion 5.0.3 has this error message "/dev/rdisk0s4"

howaa wrote: can you tell me where to find that system log?

He already told you where it was: /var/log/system.log

Just copy and paste the following command in a Terminal (/Applications/Utilities/Terminal) and then press Enter.

grep "I/O error" /var/log/system.log

It will output to the Terminal all lines in the system.log file that has "I/O error" in it.  Output such as kernel[0]: disk0s4: I/O error is indicative of a falling HDD.   If you have output such as that and aren't backed up then do it before doing anything else!