I have a 2014 11" i7 Macbook Air (8gb RAM, SSD) and a 2012 imac 21.5" (recently upgraded with 16gb RAM and internal SSD - screen off job, hairy!). Both are great machines and have handled everything I've thrown at them so far - garageband wouldn't sweat either of them. I'm not running the latest OS version as it was plagued with problems on release... The laptop cost me 400, the iMac was 300 but seems to be coming down in price now.
when my collabs send me garageband files, they open in logic and i then save them as logic files. works every time, so not sure what's happening for you. all of these projects were done on macs as well (not ios).
dang it, we only have older macs but the one i record on has the latest version, the one my hubby mixes and masters it on is older and has can only run older version, i just went to load the song into the old mac for mixing and oh dear, it wont load the file as it is recorded on a newer version of garageband. rethink required. all his presets for plugins etc are on the old mac. Both macs struggle with the files but was tryng to avoid paying for a new mac :{ didnt even think of that when i did an update a while back.
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