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Being that I have never done anything like this before, I am now wondering what to expect once I have installed the new drive.
Obviously it will be necessary to reinstall the OS, and am hoping somebody can help in giving me an idea of roughly how long it should take to install the OS (Yosemite) from a USB?
Also will it be straight forward / easy?
If anybody could please let me know that would be great and is there anything else I need to consider or look out for?
The hybrid drives have always sounded interesting to me. While I haven't used them or researched them much, I'd expect it will work out quite well for you.
After my replacement, I installed OS X 10.8.5 (Mountain Lion) from a USB stick (thumb drive). While I can't really remember how long it took, it didn't seem overly long to me -- less than an hour, I think. Most of that time you don't need to sit in front of it, either, as you can pretty much start the process and it'll reboot itself when it needs to. And while I installed onto an SSD (actually a Fusion drive setup), I don't think your installation onto a hybrid/HDD will be any slower because the real bottleneck is probably the thumb drive itself. (Unless you're installing from an external USB HDD, which should be much faster.)
Time to restore your files depends largely upon the amount of data and how you've got them backed up. It could take several hours if you've got lots of movies, for example.
There are a number of ways to install/reinstall your OS and files. E.g., you could clone your current HDD and restore the clone, you could boot from a USB drive/stick and restore from a Time Machine backup, you could (I believe) even boot
from a Time Machine backup (although I haven't tried that), you could make a "bootable installer" on a USB thumb drive to install the OS and then use Migration Assistant or just copy files from your backup.
I don't know (or have forgotten) what OS level you're currently on, and more importantly, what type of backups (if any) you have.
It sounds like you're planning to make a "bootable USB installer" thumb drive to install the OS, and get your files (if any) onto the new HDD after that -- am I right? If so, the best advice I can give you is to
TEST your bootable USB drive
before you open up the iMac -- make sure your iMac will successfully boot from the device and that the Yosemite installer starts up. If it gets to that point OK, you can be pretty sure everything will go fine once you get your new drive installed.
As far as the actual opening up the iMac and replacement, hopefully you've found a good set of instructions, with pictures. I used one from iFixit.com, and it was excellent and very, very helpful. Try to get one for your exact model of iMac. iFixit goes by the EMC number (see
their list of iMac repair guides here), which seems to be the most specific and reliable way to ID a particular machine. This site (
http://www.everymac.com/) is very useful to ID your Mac.
One hint: the glass front on my iMac came off much more easily than I expected (it's quite lightweight). You don't need to buy heavy-duty suction cups, IMHO -- I just used a couple of 1 1/2" or 2" ones like you might use to hang an ornament on a window.
Another thing I did that seemed to work out well was to gently put a layer of fresh clean plastic wrap (food storage "Saran wrap") on the back of the glass screen and on the LCD panel. (Have someone help you.) I was paranoid about getting dust/smudges on the surfaces and not noticing them until it was all assembled again. I haven't heard of anyone else doing this, but I didn't feel the need to clean those surfaces and as far as I can tell didn't do any harm.
I'm pretty handy with tools working on bikes/motorcycles/automobiles, but had very little experience with small delicate disassembly like this. I went slowly, deliberately, and carefully on each step and I was rather nervous about it. However, now that it's done, I would say it was quite easy and I wouldn't hesitate to do it again. Hope you have luck as good as I did!