fsck_hfs keeps returning "The volume was modified" I have a iMac (Late 2013) which has a standard 1TB 7200 RPM hard drive. Long story short, I noticed a dramatic slowdown in Windows (in Bootcamp) and on the Mac. After installing DriveDx, it reported that I had around 8,600 Reported Uncorrectable Errors. To my knowledge, that essentially means bad blocks/sectors. 

After looking at this post, I followed the fsck_hfs commands. I actually checked for bad blocks first using the /sbin/fsck_hfs -S /dev/disk0s2 and then ran the /sbin/fsck_hfs -drfy /dev/disk0s2 command. 

After running the check Debug, Repair, Force command five times, all it would return is "The volume was modified" which to my research says that it is not complete, and that you should run it again.

A couple of questions. Does this command actually do anything to fix bad blocks? And if not, is there another command to actually fix the problem?

Thanks.
Your disk is failing.

To answer your questions - yes, it attempts to rescue the data and mark the bad blocks so they won't be used in the future and no, there is no command to fix bad blocks.  Bad blocks are physical problems on the storage media.

The -S option, per the man page (man fsck_hfs):


  Cause fsck_hfs to scan the entire device looking for I/O errors.  It
  will attempt to map the blocks with errors to names, similar to the
  -B option.


(emphasis mine)

When you run fsck_hfs, it attempts to repair the file system - there's no guarantee that it fixes it.

In a previous post, I referenced that the hard drive in the iMac generally has a 3 year warranty (2 years after 2012) so basically, you're about 4 to 5 years into a drive expected to last about 3.  Chances are, it's failing.

I suggest replacing the drive.