Time Machine backup fails on external drive after reformatting

it appears that trying to add other things to a TM assigned drive (especially a volume) has caused your issue. storing photos on the drive has likely "confused" TM. I would recommend using the drive with TM only. and get a separate drive to store photos. the main reason for this is, when the drive fails, you will lose all your photos AND your TM. and it is recommended to have multiple backups incase the TM drive fails.

also, using seagate one touch software may also be affecting TM's functions. when you use software from just about any manufacturer for their storage device you run the risk of that same software not being compatible with macOS updates / upgrades. it can cause the software to stop working. at best this will cause you a headache. at worst it can render your data inaccessible. it is always best to use disk utility to erase and format a drive before use to remove any preloaded software.

elbaguette wrote:

Hello,

I have a 1tb seagate one touch external hard drive, sealed with a password

Noting and seconding Jeffrey's suggestion NOT to use third party disk utilities, the question is here: did you activate the "one touch" capability with the manufacturer's software/firmware, and did you "seal it with a password" also using that third party (non-Apple) disk tool?

If the answer to either of those questions is "yes," then that could well be the root cause of your problem. Third party disk tools often conflict or interfere with Apple's built in tools for managing disk drives. When encryption is involved, the result can be fatal (loss of content, or even worse loss, of access to the drive at all). I found this out years ago with a WD external drive that I used WD tools with in order to utilize its LCD display that showed the drive name, free space, etc. That malfunctioned after an OS update and I vowed never to use such third party tools ever again for drive management.

"An error occurred while starting the backup. Anti-virus software may be preventing Time Machine from writing to the backup."

Do you have any anti-virus software installed? If so, you should completely uninstall it completely.

If not, then use of those third party disk tools might have the same problematic impacts as using anti-virus software, e.g. interference with normal macOS operation.