I would backup any important data before continuing, but the issue is very likely due to the file system on the external drive. Apple and Windows only share a small number of file systems like exFAT that are fully cross compatible. Download a thesaurus for free on a mac os x bootable usb.

Macs can read from NTFS and have some capability to write but IIRC it is turned off by default and it can corrupt your drive. A better option is to use free Paragon tool from Seagate to mount an NTFS drive on a Mac.

There is a tool, that can allow you to read files off a Mac formatted drive using a PC, but if you intend to use it often I would probably just go with exFAT if your Mac is relatively new.

The native Windows file system is NTFS, which is only partially compatible with Mac OS X. Macs can read files on NTFS drives, but it cannot write to them. So if you need to get files from a PC to your Mac, NTFS is a decent option. Microsoft NTFS for Mac by Paragon Software is the best such software solution: reliable, fast, and now affordable as well Fast, seamless, and easy to use, Microsoft NTFS for Mac by Paragon Software is required for those who need to write to Windows volumes. And now it’s more affordable than ever. MacWorld, USA 2016, macworld.com. Frequent logout for google account in mac.