My Seagate Backup Plus Slim disk box states it can be used with both Windows and Mac. It worked fine with the Power Mac and also with the Windows machine. However, after I used it on a Windows machine, it appears as Read Only on the Mac. Other than formatting, what can I do to regain write capability with the Mac? I would like to use the disk to move files back and forth between the Windows and Mac machines.
"Using a Mac is a wonderful experience – until you try to use a NTFS-formatted drive"
I believe that your external hard drive is currently NTFS formatted. Sadly, macOS doesn't natively provide write access to NTFS formatted media. You have to resort to 3rd party drivers to get write access to their media. There are at least two commercial NTFS drivers available:
However, for your drive, Seagate seems to offer a NTFS driver free of charge:
Download and install this driver on your Mac. It should give you write access to your NTFS formatted hard drive without reformatting.
I had the same problem and this fixed it instantly.
Reinstall the NTFS for MAC OSX (if you don't have it yet). You can download the driver here : http://www.seagate.com/as/en/support/downloads/item/ntfs-driver-for-mac-os-master-dl/
From the details given, the windows machine most likely formatted the new hard drive with an Ntfs partition. Ntfs has read-only support in OSX 10.3 and above, but you may be able to find some software that allows for OSX to read and write ntfs, such as NTFS-3G.
It seems there is no easy "thumb-drive-like" way to use a hard disk to switch between Mac and Windows machines. I finally had to reformat the unit and will use it with the Mac only. There are in fact, several OEM SW products to enable the function but most seem to require technical expertise that goes beyond what I want to get into and have no integrity guarantee. I will probably use a 32 GB SD card as Exchange media between machines. Still can't uncerstand why isn't it posible to work with hard disks as well as I work with SD cards or thumb drives. And, BTW Seagate should not put the misleading legend on their disk boxes, it must explain the "between" works only in read only mode.