High Sierra & External Hard Drives (toshiba)

Your drive is likely formatted NTFS for Windows. You have been running a third party app such as Tuxera or Paragon to enable you to write to the drive. You updated your OS without updating this third party app so you lost the ability to write to the Windows formatted drive.

Either format the drive for Mac or update your third party app. If you choose to format then make sure you copy off all your data since it will be lost during formatting.

The external hard drive is compatible with both windows and Mac.

Not any more. How is it formatted? To determine that use Get Info like you did before. Its format is shown adjacent to Format:

Refer to this Discussion: I can no longer write to or delete from my external hard drive. can anybody help to solve this problem

NTFS isn't an option in Format a disk for Windows computers using Disk Utility on Mac - Apple Support. macOS can read, but not write, NTFS formatted volumes. That's the reason you can't copy anything to it.

I don't use "Tuxera" so I can't offer any advice other than to let you know those things typically break with macOS upgrades.

You can use literally any external USB hard disk drive with a Mac. Buy one big enough for your needs. If you don't know where to start, I suggest shopping for one from a vendor known to support Macs such as OWC / MacSales. You don't have to buy one from them, but if you have questions or problems with it at least they won't slam the door in your face saying they don't support Macs.

The very first thing you should do with a hard disk drive is to erase it so that it can be used with your Mac, and if you also need to connect it to a Windows PC, choose a format that will work with it also. That will also erase whatever useless software or utilities hard disk drive manufacturers are fond of bundling with them. To learn how to do that please read Erase a volume using Disk Utility on Mac - Apple Support.

If you want the same hard disk drive to be used for both Macs and PCs, then format it using one of the options common to both operating systems: Format a disk for Windows computers using Disk Utility on Mac - Apple Support

I also have same problem , and need to buy is 31$

I am decide to buy new External copy everything to new HDD and formate old HDD .

because 1T of HDD only 65$ and software price already 50%

so better buy one HDD . it good option for me better then buy NTFS from Tuxera