Probably the formatting of your usb drive, the file system. You could use fat32 (not so good anymore because of limitations), or preferably exFAT. NTFS can't be written to on a Mac, only read from. If the stick was to be formatted for Mac, a.k.a HFS+, you could not have read it at all with a PC.
There are third party extensions for Mac which do allow NTFS writes, but I do not know their status right now. There are third party programs for windows too to read HFS+ format, but I think your, and everybodys, best solution would be exFAT.
So, erase and/or format USB-drive to exFAT, and you should be ok with cross platform transfer again.
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