Time Machine error when trying to backup

[Itzamna]

Hello there. So, I had to format the harddrive where the Time Machine backups were stored. After that every time I try to backup, it stops right in the beginning and it says that the device is unplugged (which it isn't). I checked the Time Machine log and it gives an error 37.

After a few tries, I discovered that if I unplug another usb external hard drive, the backup concludes without any problem. Also if I connect that drive to the USB-C port through an adapter, the backup concludes with no problem. It only gives the error if both hard drives are connected to the USB ports. I also have a third external hard drive, but that doesn't interfere...

So, what gives? Any ideia on how to fix it?

The TM drive is Mac OS Extended and the other drive is APFS with 3 volumes.

Thank you

 

[Taz Mangus]

Eject and disconnect all hard drives and try a SMC reset.

Reset the SMC of your Mac - Apple Support

The SMC manages power and hardware behavior on your Mac. This troubleshooting article explains when you should reset it, what issues it can fix, and how to perform an SMC reset depending on your Mac model (Apple Silicon, Intel with T2 chip, or older Macs)

support.apple.com

 

[Itzamna]

Eject and disconnect all hard drives and try a SMC reset.

Reset the SMC of your Mac - Apple Support

The SMC manages power and hardware behavior on your Mac. This troubleshooting article explains when you should reset it, what issues it can fix, and how to perform an SMC reset depending on your Mac model (Apple Silicon, Intel with T2 chip, or older Macs)

support.apple.com

Thank you but it didn't work.
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I deleted one of the volumes from the other hard drive and all of the sudden it works... :/
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I re-added the volume and.. It works normally... This is strange. Why does a APFS volume of another hard drive blocks the backup?