Maurice wrote:

In one of my checks on W7 with the .MTS folder, it offered to 'fix it',
and after checking
the result I found that it had simply changed all the "*.MTS" to "*.CHK"
and renamed the
folder "FOUND.000"!

No, it did not change the extensions of the files. CHK files are
recovered file fragments due to a corrupted file system. chkdsk could
not ascertain for sure to which original file an orphaned cluster
belongs so it reallocates the cluster into the file system giving it a
generic fileXXXX.chk filename. CHK files are lost/orphaned fragments of
files that are protected from overwriting by allocating them to new .chk
files in the existing file system. You should run 'chkdsk drive: /r'
on that flash drive. It has a corrupted file system, or had a corrupted
file system if the "fix" was to run chkdsk.

https://www.howtogeek.com/282798/wha...le-in-windows/

Note that chkdsk might fix problems now but flash drives operate
differently than do magnetic media. If the reserve space is not getting
used to mask bad sectors on the flash media so no bad ones are ever
found, the flash drive is going bad. The firmware inside the flash
drive responsible for wear levelling and masking should make the drive
look always look good until the reserve space gets used up, there's no
further space for masking bad memory blocks, and the drive
catastrophically fails. If a flash drive starts to go bad, chkdsk might
be a temporary cure but I'd get the files that I could off that drive
and discard it.

https://www.raymond.cc/blog/how-to-c...b-flash-drive/

You can find all sorts of flash drive test tools. Most will write gobs
of data to the drive. Flash drives have a maximum number of write
cycles and such testing reduces their lifespan. Because flash has a
limited write cycle durability, they will die. Sometimes suddenly,
sometimes getting flaky but will get worse and then die suddenly. If
you write a lot to flash media, consider it disposable media.

If chkdsk fixes a flash drive, hopefully it was a logical structure
problem only in the file system. If the errors were due to media
defects, look at getting the files onto different media.

You might also consider what file system is on that flash drive. FAT
has no error recovery for files that were left open when the OS crashed
or the processes were killed that had files open on that media. NTFS
includes journaling which can be used to rebuild the cached/buffered
data that failed to get written to the media. FAT is more portable but
NTFS is safer.