Why a full rescan?

[pka4916]

1 thing that is really annoying is the following

I have an SSD 2 Tb and an external USB HDD (3.0)
When I backup the missing files from the drive. the 1st time it takes 10 min. to scan and then it's backing up.
I canceled several times, and did a rescan, and it takes seconds, and continues where it left of.. Great :)

Now, when I disconnect the USB drive, and do a sync again, it's rescanning all the files again..
Why is that? shouldn't it just take a few seconds again? since there are already files added during the sync
the scanning takes much longer, and currently over 20 min.

If this is happening every time when I re-connect my USB drive (same drive letter btw)
then it's really becoming a pain in the butt, to use this program.

Is there not a file that keeps track of everything? Like other programs do?

[Zenju]

You're seeing ordinary I/O buffering done by the OS, not FreeFileSync.