On the desktop, the Seagate drive shows up as "SCSI0_VOL1. When I Ctrl-click that icon and Get info, under "Ownership & Permissions:" it says, "You can only read." Also, when I try to move the folder "Micah" onto the SCSI0_VOL1 icon, an error message displays, saying "The item 'Micah' could not be moved because 'SCSI0_VOL1' cannot be modified."
Cancel question. I figured it out while I was waiting.
I found a user's manual for the Seagate drive at Seagate' website, and it told me that you have to format it for use with a Mac or a PC, but not both. As I had previously used it to back up a PC, I just reformatted it, and then the Mac saw it as a writeable drive.
Thanks for your help.