Miker wrote:

Looking for a way to see if files are missing on a flash drive as
compared to my hard disk folder and then be able to copy to synchronize.

Called "mirroring".

Any good free applications for that ?
Would be nice if it worked on Windows XP but not mandatory.

SyncBack Free.

System requirements: Windows 10, 8, 7 and Vista (32/64 bit). Lots of
program quit supporting Windows XP which died 5 years agao (April 8,
2014) after being supported for 12 years. Sorry, no support for XP.

I paid for SyncBackLite (no longer available, so the next step up to
payware is SyncBackSE) because it can use VSC to copy locked files. The
free version can't do that, so you see access errors in a job's log.

Another is FreeFileSync, but I found SyncBack is easier to use.
FreeFileSync supports Windows from 10 back to XP, along with macOS, OSX,
and various Linux distros. However, there is a note on their site
stating that FreeFileSync will support Windows XP and Vista only until
version 10.11. The latest is 10.12, so you have to find the older
version.