I’ve seen extreme sync times when writing large files to an old slow USB thumb drive. Often you will see the first part of the copy run at full USB speed but t

I’ve seen extreme sync times when writing large files to an old slow USB thumb drive. Often you will see the first part of the copy run at full USB speed but then it slows to a crawl. Then the unmount (which does a sync) pays back that initial burst of speed. I’ve seen it take more than 5 minutes (a kernel warning about the slow sync appears in dmesg after 300 seconds IIRC).