Unable to Add Files

I purchased an external drive to get my personal files off my computer at work when I left there. But now, I cannot add anything else to the drive from my home computer. My work computer was a PC and at home I have a MacBook Pro. Would that be the reason? I have only used 1.99 TB left on the drive and it will be a complete waste if I an unable to do it. I have read some of the info on here about changing the properties, but I don’t even have the option to view the properties. Thanks for any help!