What Is a .CHK File?
You found FOUND.000 folders full of .chk files after a disk check. They are recovered fragments. Here is how to tell what they were.
What a .chk file is
A .chk file is a fragment of data that CHKDSK recovered from a corrupted or improperly ejected disk. Windows stores them in folders named FOUND.000, FOUND.001, etc., with names like FILE0001.CHK.
What is inside
Each .chk holds bytes that were part of a real file whose directory entry was lost. It may be a whole small file, part of a large one, or unusable junk.
How to identify one
Open it in a text editor to look for readable content, or use a file-type detector / hex viewer to spot a signature (e.g. a JPEG or ZIP header). If you can identify the type, rename it with the right extension and try to open it.
Is it safe to delete?
Yes, if you are not missing any files. The .chk files and the FOUND folder are recovery leftovers.
FAQ
Can I recover a .chk file?
Sometimes. Identify its true type with a hex viewer or file-type tool, rename it with the correct extension, and try to open it.
What created the FOUND.000 folder and .chk files?
CHKDSK — Windows' disk-check tool — saved recovered fragments there after finding lost file data.
Is it safe to delete .chk files?
Yes, if you are not missing any files. They are recovery leftovers.