Check the health of your hard disk, use HDDScan or any other software that can extract SMART info from your hard disk, then look at few of these attributes with keyword "sector", such as "Reallocated sector count" (worst possible case), "Pending sector count".
If there's any number other than "0" then backup all files from that hard disk to other hard disk. Look for software that can wipe your hard disk, usually format won't be enough to clear the "Pending sector count" flag, if the "Reallocated sector count" flag is increasing after you do wiping, proceed to RMA your hard disk, you can't rescue such severe damage.
This is an usual sign of failing hard disk, your case may be different, but you may try out those method as I faced this quite few times from my own hard disk and my time as technical support.

Check the health of your hard disk, use HDDScan or any other software that can extract SMART info from your hard disk, then look at few of these attributes with keyword "sector", such as "Reallocated sector count" (worst possible case), "Pending sector count".
If there's any number other than "0" then backup all files from that hard disk to other hard disk. Look for software that can wipe your hard disk, usually format won't be enough to clear the "Pending sector count" flag, if the "Reallocated sector count" flag is increasing after you do wiping, proceed to RMA your hard disk, you can't rescue such severe damage.
This is an usual sign of failing hard disk, your case may be different, but you may try out those method as I faced this quite few times from my own hard disk and my time as technical support.

Hey there Homura15

Just would like to double confirm, can you copy the file to a different location (not to the external drive?)

Most likely there is a problematic file, or problematic portion of the disk that is causing this error then. Like mentioned by wildwestgoh I would suggest running a chkdsk on the machine, which might fix a problematic file issue and might be able to correct a minor disk error.
How to run Chkdsk from My Computer or from Windows Explorer
Double-click My Computer, and then right-click the hard disk that you want to check.
Click Properties, and then click Tools.
Under Error-checking, click Check Now. A dialog box that shows the Check disk options is displayed,
Use one of the following procedures:
To run Chkdsk in read-only mode, click Start.
To repair errors without scanning the volume for bad sectors, select the Automatically fix file system errors check box, and then click Start.
To repair errors, locate bad sectors, and recover readable information, select the Scan for and attempt recovery of bad sectors check box, and then click Start. Note If one or more of the files on the hard disk are open, you will receive the following message:
The disk check could not be performed because the disk check utility needs exclusive access to some Windows files on the disk. These files can be accessed by restarting Windows. Do you want to schedule the disk check to occur the next time you restart the computer?
Click Yes to schedule the disk check, and then restart your computer to start the disk check.
I would recommend Running it to repair errors and locate bad sectors in this case.
Hope this helps.