ubuntu system freeze while diff reading, SysRq REISUB not working

I've experienced total system freeze while checking if files where copied correctly by

diff -rq [INTERNAL HDD] [USB HDD]

This happened twice, always while running the diff. On the second occasion I had system monitor up and RAM was about 30% filled and CPU was also empty. I've tried Alt+SysRq+REISUB to reboot safely but the system was totally unresponsive. Suspecting RAM, I've conducted memtest without any issue. Do someone have any idea while this happens? (I'm a bit reluctant to test it further much as the data are valuable). The size of the data is ~4T in ~60k files. Some system info

INTERNAL
description: EXT4 volume
capabilities: journaled extended_attributes large_files huge_files dir_nlink recover 64bit extents ext4 ext2 initialized
configuration: ansiversion=5 created=2024-02-28 16:06:38 filesystem=ext4 label=18T lastmountpoint=/media/marek/18T logicalsectorsize=512 modified=2025-03-20 08:38:08 mount.fstype=ext4 mount.options=rw,nosuid,nodev,relatime,errors=remount-ro mounted=2025-03-20 08:38:08 sectorsize=4096 state=mounted

# for some reason, lshw reports this hdd to be "size: 380GiB"
# while lsblk reports (expected)
# sda                        8:0    0  16,4T  0 disk   /media/marek/18T


USB
filesystem=ntfs label=Elements mount.fstype=ntfs3 mount.options=rw,nosuid,nodev,relatime,uid=1000,gid=1000,iocharset=utf8 name=Elements state=mounted


SYSTEM
Ubuntu 24.04.2 LTS
Intel® Core™ i7-5820K × 12
RAM 32,0 GiB
Linux 6.8.0-55-generic