Unable to backup files in use by the operating system.

I’m having trouble with files that are in use by Windows. Checking for the files in use in Process Explorer, many are being used by the SYSTEM process. I’ve tried setting the backup program to use raw read to copy the in-use files, but it is not working. I am getting messages “VSS is supported only on NTFS volumes!”, “Skipping inaccessible item (filename). Error code 90” on some files, and on other files “Unable to process item (filename).” I’m copying files from a FAT32 hard drive to an exFAT SDXC card, no NTFS drives are present. I’ve tried disabling the volume shadow copy service entirely, but the VSS message still comes up, even though it seems irrelevant.

Comodo Backup 4.4.1
Windows XP SP3 32-bit
Avast Antivirus 2015 (turning off the file system shield didn’t seem to help)