Moved my time machine backup drive to another computer, can see it but won't recognise old backups I've been using an external hard drive on my MacBook for time machine for a year or so. Now I have a Mac Mini too and have connected the external hard drive to that for the same purpose.

I want to leave it connected to my Mac Mini and still backup to it wirelessly from my laptop.

I have done all the steps to allow time machine to recognise network drives and I can see the drive from my laptop etc., but it won't recognise the old backups. 

Do I need to change the file permissions or similar to get it working?
Just Just go into System Preferences … Time Machine and hit Select Disk… 

(If that's not working, let us know what error you're getting/what does happen.)