Time Machine in Catalina breaking secondary backup disks

For years I've been backing up my mac to two separate hard drives in Time Machine. Until installing Catalina this worked great. About two months after installing Catalina the second drive in the list just died without warning - not the usual hard drive fail, this was some kind of unrecoverable hardware problem. I wrote it off to the drive being old, and got a new one. Now, a month later, the replacement drive (a sandisk extreme ssd, 2gb) has stopped working in Time Machine - the error is something about not being able to create the backup folder, or An error occurred while copying files. I've tried reformatting, and first aid, several times - there don't seem to be any problems with the drive according to Disk Utility. But each time I re-add it to Time Machine as the second drive, it throws the same error and stops working.