Scandisk won't fix the disk.  It might fix the file system, but the 'fix'
might be worse than the problem, and in any case it won't succeed if the
disk is still faulty.

Undo simply enables you to restore the drive to the unfixed state, and since
you already know that's no good, there's not much point.  The slowness is
probably Scandisk re-reading the sectors to try and get them to come out
right.

If the drive is not faulty then perhaps the problem was a corrupted
parameter table of some sort, which can lead the OS (and scandisk) into
thinking that the drive is completely screwed.  Since you have let Scandisk
write to the disk while it was in that state, a fresh start is probably the
only option.

Not sure why you would attempt a Scandisk undo anyway, though likely that's
not the issue you should be addressing or how it should be [have been]
addressed. Unless your an expert at disk and file recovery, the files
created are essentially useless.

Scandisk uses the MBR and fat tables during its disk review. If both or
either of those become corrupted then the disk APPEARS to be corrupted,
hence endless read, re-writes, fat creations, creations of recovery files,
and more failures because of the mis-matches.
You state Seagate's tool found no hardware fault with the target drive:
good, then that means your issue likely does not mean disk failure.
Moreover, you state your other drives/partitions show no file structure
problems, meaning the issue is localized to c: drive/partition. That also
appears to indicate that its not related to memory errors, or other BIOS or
hardware issues.

Had you NOT tried the Scandisk with undo, it might have been possible to
correct the errors by re-instituting a good MBR and fat. Now that is likely
impossible. It still might be useful to try though, perhaps allowing
recovery of some files that might be irreplaceable, though it is generally
recommended to stop accessing the drive/partition until you create a
complete plan for recovery.

You could still try a real recovery tool such as testdisk, HDAT2, or other,
and try a recovery; though again, the undo used and your indication of
re-writes might have negated even that. Fats will not match, nor will actual
file locations on the drive/partition. If it is possible to get to another
usable OS on your computer, or another computer in which you could install
that drive, then recovery could [might]possibly effected.

The short is:
Scandisk is a useful tool for "indications" of issues, and somewhat useful
for correcting minor issues related to improper shutdowns, but for real work
on a drive, IMO, it is best to diagnose the causal issue and use some other
tool for the correction.

Having used now Scandisk with undo, you may now have to wipe that
drive/partition [after recovering what files you can] and re-install
whatever was there. Moreover, the apparent dual boot issue [boot
management], will require special handling related thereto.

Determine exactly what you would like to accomplish, post what that is and
how you intend to accomplish that, and attempts to help address/achieve that
will be provided by the group.