Hi,
If that external drive is over 4 years old; it's probably failing or has failed. Stopping the Chkdsk may not damage it, but you should test that drive using drive diagnostics listed here:
Link Removed.
If you don't get through the Chkdsk in 24 hrs. you should stop it. Unless you have a hdd larger than 2TB or something. You should know that if you don't have another backup of the stuff on that drive, continuing to run further diagnostics on that drive can reduce your chances of getting data off of it before it completely fails. If you
DO have backup of some or all of that data elsewhere, you can continue fooling around with it taking the risk of killing the drive completely or not.
If you
DON'T...