I've been using flash drives on my Raspberry Pi's for Samba shares. Whenever I tried to copy large files to a share from a Windows machine it would hang for a bit and eventually fail.  I tried reformatting a flash drive to exfat (vs fat32.) Still the same issue.  Interesting enough, this issue doesn't occur with Linux clients. I eventually found this thread. It mentioned the underlying issue being using exfat/fat32, and to try using ext4. So I did. It worked! Everything now transfers fine, big or small.