Help! Can't unlock Photoshop file

I'm working with a photo I edited on my iMac, then stored to an external HD to work on it on my MacBook. But when I try to save it it says "File is locked because you don't have the necessary privileges." I went into Get Info and changed all the Permissions to read & write, but it still won't save. Help!

TRy editing the permissions of the whole drive and select apply to all enclosed items (from the cog type icon) or select the whole external drive and select ignore permission son this drive from the get info window