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Is it formatted as FAT32 ?
If it is formatted as NTFS, then your Mac will not be able to write to it. Mac can only read from NTFS by default. But if it is formatted as FAT32, then your Mac can read and write from and to it. But you will not be able to write files that is over 4GB in size in FAT32 format. If it is formatted for Mac (HFS), then PC will not be able to read or write it.
But there is a way to overcome this problem. Format the flash drive as NTFS, then install this application on your Mac --> https://www.paragon-software.com/home/ntfs-mac/
That application will install some drivers that allow you to write to NTFS formatted drives. Just use the trial version (no need to buy it). That is all you need to do to copy the file to the flash drive.
If you have any questions or would like any additional information, please don't hesitate to ask.