Hi there! Because the forum you originally posted in is for beginners trying to learn the basics of Photoshop, I moved your question to the Photoshop General Discussion forum, where you'll get more specialized help.
To help others help you, please read through this article and provide any additional relevant details.
When you write files to a disk from another computer the new computer may not claim ownership so creates a permission problem. Do a web search for drive ownership Mac. As a workaround believe if you click on drive at bottom there is a box to "ignore ownership".
Mac OSX can read but not write to Windows NTFS partitions. You may try out a third party utility such as Paragon NTFS for Mac or explore other options like moving the files to a folder on your Mac or an external HFS+ or FAT formatted drive.
Update: I installed Paragon NTFS for Mac on my Macbook and think it's a great idea if you have several external NTFS drives. Costs $20 but you can get on with your work a lot more smoothly.