I upgraded my iMac to Yosemite without difficulty on Friday evening. Took an hour or two to download. Has been rather less easy on my 2010 MBA. Download has had many false starts and predicts anything from 10 hours to 2 days to download. Now it may be that my ISP have throttled my broadband speed due to all the data I've been downloading with all the recent updates but I don't think so. Other wifi tasks seem to be ok. Are the Yosemite download files generic or machine specific? Will my iMac download file still be on the machine to be able to copy across to the MBA if it is good for updating my MBA? If so, where is it located? I do have a USB Ethernet cable somewhere if that's an alternative to downloading via wifi, though I'm assuming that wouldn't be much quicker.