I have numerous external drives all formatted as FAT 32 on the Mac which transfer Quicktime back and forth between Windows and Mac machines.
As an additional note, check that the CODEC used for these files is available on the PC or they wont play.
We encode our client previews as H264 for playback only.
Applications > Utilities > Disk Utility > Partition >
in volume scheme select 1 partition
format: MS DOS FAT
options > Master Boot Record
apply and it will partition and format the drive