Can't write to Seagate external hard drive on other macbook pros I have a "new" 2gb Seagate external hdd.  I used it to copy files off of my old Macbook pro, circa 2014, which worked fine.

I just plugged in the drive into my new Macbook pro, circa 2018.  I can read files off the external drive.  However, I can't copy files onto the drive from the new Macbook.  I grabbed my friend's Macbook (circa 2014 so same model as my old laptop) and same issue, I can read files from the external drive but I can't copy files onto the drive.  I don't have my old laptop anymore so I can't try to copy things to it.

I plugged the hdd into a Windows 7 laptop, and I'm able to copy files onto the drive, create a new folder, etc.

I found several links that said I need to reformat the hdd to make it usable with my Macbook but it makes no sense since my old Macbook was able to read/write to the drive without being reformatted but now with any other Macbook, it doesn't allow me to write, only read.

What happened?  How come I can only read files from the hdd from any new Macbooks but a Windows laptop I can read and write?  Why does it seem like the drive is married to my old Macbook?  If it is married to it, how to I break this bond so I can use it with my new Macbook?
If the drive is NTFS the MacOS default driver is read/only. 

You probably have installed a driver at one time on your old system that provide read/write. 

Such a driver frequently come with external harddisks. I have personally found this preloaded on recently purchased Seagate drives formatted as NTFS, but that only supported Seagate drives.  Paragon sells the full driver from https://www.paragon-software.com/home/ntfs-mac/ (they also have for other filesystems).