Are you referring to the write buffer?
If so, look into the sync option when using mount or the fstab.
However, I think it’s best to consider this external USB drive as dead. I wouldn’t trust it to keep my data safe.
If you’re referring to the drive’s own internal cache, then I’m not sure that’s possible, since it’s at the firmware-level.
Again, regardless, not worth it in my opinion.
That doesn’t matter. Digital storage and disk drives are not like lightbulbs.