You must have a USB key or installation CD's.
I bought the new hard drive, installed it, but now it won't boot up to OS Mountain lion(dvd) I downloaded from Apple .
Do I have to format disk first, if so, how do I do that?
You must have a USB key or installation CD's.
WRONG. Absolutely completely WRONG. You should NEVER trust the cloud as the only storage location for your data.
Store your data on your local disks, and BACK UP to the cloud, or to other local disks. Anything in the cloud can go away any time and there's absolutely nothing you can do about it. Look at Geocities, MobileMe, and other cloud services that have shut down over the years.
Trusting the cloud is the worst thing anyone can do if they care about your data. It's actually worse than storing your data on a single hard drive with no backups.
I bought the new hard drive, installed it, but now it won't boot up to OS Mountain lion(dvd) I downloaded from Apple .
Do I have to format disk first, if so, how do I do that?
You actually have recovered from a Time Machine backup? lol.
is there a different purpose for time machine i'm not aware of? Not that i use it.
Try an external enclosure for the hard drive and connect it to the iMac. It should show in finder if you can access the files and then you can just copy over the files you need. I hope it goes that smoothly for you, anyway.
You actually have recovered from a Time Machine backup? lol.
Luckily with iCloud (which granted, has little space), backs up to each computer you're signed on with.
I don't think iCloud backs up computers, only iOS devices.
Indeed. I love storing all of my stuff on unencrypted cloudy storage platforms. In addition, uploading TBs of data is so quick that it's really convenient.
/sarcasm.