Is a tool available to check the integrity of copied files?

[Ralf Mardorf]

Hi,

I've done some googling for "does linux diff compare data using a cache"
to figure out what's going on. The first diff of 10 GiB of
data copied from a SATA3 SSD to an USB 2 stick connected to an USB 3
port took around a minute, right after the copy finished. A second diff
needed 3 seconds. Both returned exit status 0.

It's impossible to read 10 GiB of data in 3 seconds from an USB 2 stick.
Does diff use cached data instead of comparing the "real" files line by
line?

Google returned "diff isn't doing any caching. The OS is. If you are
using Linux, you can flush the disk buffers and cache".

I expected that diff ensures to compare the "real" files line by line,
but seemingly diff isn't aimed to check integrity of data.

Does a command exist that compares "real" files, not just cached files
by default?

I experience weird things with Raptor Lake hardware, especially if USB
is involved and I want to check the integrity of USB transferred, saved
files by using a tool, without manually clearing cached data manually.

Usually I don't sent the same request to more than one mailing list, but
this time I urgently need help, because I run from one into another
issue with new hardware. I've made harmless careless mistakes that were
time-consuming because I'm overworked. Maybe I can't see the forest for
the trees.

Regards,
Ralf