Depending on how you format it, it may also cause problems if you need to use the drive with a Windows PC. This will wipe all the data from it, so you should make a backup or manually copy important files to another location first. One is to reformat the drive to a format your Mac can write to as well as read. There are several options, depending on what you identified as causing the problem. The following applies whether the drive in question is a hard drive, USB stick, SSD, or any other external media. If it says “read-only,” you know the problem is permissions-related.
So here's a tip for you: Download CleanMyMac to quickly solve some of the issues mentioned in this article.