Windows 7 - system partition changed filesystem from NTFS to RAW

Hi all,
in last few days I noticed strange behavior in one company. I manage IT there. They have about 100 client computers, most of them are Windows 7 Professional, the rest are Windows 10. Most of client computers are notebooks HP EliteBook.

In last few days on 3 notebooks appeared problem with booting. Windows did not boot and BSOD screen was shown. Error message was: "C0000135 Program can't start because %hs is missing. Try reinstalling the program...."

When i went into Windows recovery environment I noticed that system partition was changed from NTFS filesystem to RAW. Why? I don't know.

There is no hardware problem. Hardware test finidhed without any errors, after reinstalation notebooks works OK.

What is strange is that all 3 affected notebooks were bouhght in the same time (middle of 2014).

Configuration:
Windows 7 Professional, original instalation from HP, bloatware uninstalled (except HP Connection Manager and drivers). System partition is encrypted by TrueCrypt. There is centraly managed antivirus Avast, system is full patched from internal WSUS server.

Any ideas?

Thanks
Bohdan