"The best solution is to keep the drive attached to your iMac and in system preferences -> sharing -> files sharing add privileges to your drive for your user. on your MBP go to the time machine preferences -> choose a drive and you will see the USB drive attached to your iMac. keep in mind that to back your MBP your iMac and your USB drive will both need to me on."
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Hi andyboutte, thank you very much for this advice. I can surely use this in my own setup. Have a nice day!🙂