Since the external hard drive keeps disconnecting issue occurs only after upgrading to macOS Big Sur, it's highly likely there are some system bugs that prevent your drives from functioning normally. Another possibility is your drive doesn't support the upgraded OS. To fix the disconnecting issue, you can choose to upgrade or downgrade the macOS. However, before that, it's necessary to recover data from disconnecting external hard drives.
If your Seagate external hard drive keeps disconnecting while backing up or transferring data, your data can easily get lost during the process. To avoid further data loss, you should use professional Mac data recovery software, iBoysoft Data Recovery, to recover data from external hard drives on Mac.
Due to its high recovery rate, various applicable scenarios, and a wide range of macOS support – from macOS 10.9 to macOS 13 Ventura, iBoysoft Data Recovery for Mac is one of the best external hard drive and USB recovery software. Download it to experience excellent data recovery performance.
Now we can proceed with solutions to fix the external hard drive keeps disconnecting on Mac problem.
To start with, run a basic combability check of your external hard drive. Go to Seagate official website and see if your model works with the OS you install. If yes, as mentioned above, the biggest culprit so far is the macOS itself. Therefore, you can either wait for the next update and hope Mac engineers can fix this issue. Or, you can manually downgrade to the previous macOS. If not, the only option left is to downgrade macOS.
The process to downgrade macOS requires full preparations. You may refer to the link above for a detailed and comprehensive tutorial.