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It can be really frustrating when you can't drag and drop files to an external hard drive on Mac. There are several possible explanations for the issue, but in most cases you'll be able to solve the problem after a little bit of investigation and troubleshooting. In rare cases, the external drive may remain unwriteable, but usually – even if you have to format the drive – there is usually some solution that will work. The first step is checking the hard drive's cables and making sure everything is connected properly, but if that isn't the issue, there are several factors to consider.
Open the Disk Utility software in your /Applications/Utilities folder. Click on the drive in the list of drives in the left-hand side of the window. Then check the bottom of the window for the entry "Partition Map Scheme." Mac OS X cannot write to disks that are formatted using NTFS partitions, a common format used by Windows computers. There are various methods for enabling this capability (for example, TEQTA offers a couple of free solutions), but they require expense or technical expertise; it is easier to copy all files off the NTFS-formatted drive, reformat it for Macintosh use, then copy the files back.
If the formatting of the drive is correct but you still can't put files onto an external hard drive on a Mac, it might be a problem related to the permissions on your drive. You can easily fix this by going to the info screen for the drive, as described by EaseUS. Right-click the drive and choose "Get Info" to bring up the information window. Expand the section that says "Sharing & Permissions," then click on the lock icon so it's open.
You'll see a list of names and privileges above the lock icon, and this may be the source of the issue. Either you can choose individual users to grant "Read & Write" access to, or simply allow it for everybody. You can also click the check-box beside "Ignore ownership on this volume" if you want to fix the issue as simply as possible. In any case, after completing this process you'll be able to transfer files to the drive as you ordinarily would.
If this approach doesn't work and you find you still can't save to an external hard drive on Mac, then you can try the other methods in this article, or simply format it as a last resort.
The simplest reason why you may not be able to write to a hard drive is if it is full. Single-click on the disk in the Finder, then choose "Get Info" from the "File" menu. Check to make sure there is space listed after the label "Available:" in the General section of Get Info. Also check the size of what you're trying to put on the drive. Even if you have space on the drive, if the file you want to write over is larger than the space available, you won't be able to write it to the hard drive.
Disk Utility can try to verify or repair a drive that cannot be written, although if the drive remains unwriteable to that software as well, repairs will not be possible. You can also try copying the files off the drive and reformatting it to see if you can fix the drive in that way. However, if you see multiple errors on the drive during a Disk Utility check, it is safer to retire the drive and replace it with another one; a problem with the hardware cannot be permanently fixed with a reformat.