Still not ready? See https://www.cnet.com/forums/discussions/running-chkdsk-on-external-hd/
It will be much faster if you mount it as an internal hard drive instead of external. So maybe wait for a while and then abort it and do that.
There's no obvious relation between failing to boot and not being able to access other files. Just checking for file system errors would have been enough.
Then copy what you need and that's certainly not all. If the normal Windows or Linux programs fail at certain files, use roadkil's program from http://www.roadkil.net/program.php?ProgramID=29 . You could even try that when it's still mounted externally.
Things like this are why most people make a backup of files they don't want to lose. Much easier than recovering and usually with more success. Didn't you do it?
I am running chkdsk on a 320 GB externally attached hard drive. I selected both "automatically fix file system errors" & "scan for & attempt recovery of bad sectors". It has now been at "37531 files processed" for about 18 hours. No error messages and both the internal & external drives show signs of activitty. Is it hopelessly stuck or will it eventually move forward if the current file is irretrievable? Note: the externally attached drive was an internal drive that failed to boot and was replaced. There is data on there that I really need to retrieve. Are there better options for retrieving all salvagable data?