"Suddenly I can't save files on my external hard drive anymore!"に関する回答https://www.boxaid.com/blog/fix-read-external-hard-drive-mac/ For those still struggling: I have a Seagate External HD that was never partitioned for Mac as I use it to share files between Windows and Apple computers. Suddenly I wasn't able to write/copy files to the HD (maybe due to upgrades of my MacOS?). Followed the advice of this website and downloaded the recommended helper for Seagate. Very helpful.