I was doing a copy of my savefile 4fs from my USB FAT32 to SSD
mostly done then boom
sector errors

dmesg -w

now BW64 won't load the savefile
grub looks good

Can I fsck the 4fs or some other tool?

USB seems fine
did an unmounted fsck on it, found lost chains ...
badblocks all ok

Was there a read/sync issue?

opinions

Try this:

Use e2fsck

The save file can not be in use.

Mount the partition the save is on.

Open a terminal, and enter:

e2fsck /path to savefile

example:
save file is on partition sda1 and named pupsave.2fs.

e2fsck -p /mnt/sda1/pupsave.2fs

The -p option says to auto repair anything found.

To see other options for e2fsck,
enter e2fsck in a terminal

Fsck has a utility for checking vfat formats, but is not as good as the programs in Windows OS for doing this.
Really best to check fat32 format using the programs in Windows. (chkdsk and defrag)

Anytime you do a file system check.
Keep repeating it until no issues are reported.
Any program may need to do the check several times before it gets everything corrected.