External drive back up failure
Just got a new iMac using Catalina - trying to set up an external drive for a time machine back up and I keep getting this message "POSIX reports: The operation couldn’t be completed. Invalid argument."
Then the disk disappears off the desktop. I first thought the external drive was faulty was I swapped it but it has done it again. Wow. this used to be straight forward. Any help would be appreciated?
iMac 21.5″, macOS 10.15