I have an iMac with a fusion drive. A while back, the drive separated into two seperate drives, with one OS on both the SSD and HDD each. The Apple Technician was unable to help me to join them again into a single fusion drive over the phone, and so I gave up and just used the OS on the HDD, while rarely using the speedy SSD for importing and rendering video only. However, the 3TB HDD is nearly full, and thus is running incredibly slow – almost unusable. Is there any way for me to delete the unused OS from the SSD, and move just the OS from the HDD to the SSD, while still leaving the data on the HDD. I have 2.8 TB worth of data on the HDD, and only 200 GB free on the SSD, so is there a way to move just the System OS to the faster drive, while leaving all of the data on the HDD without merging them into a Fusion Drive again?
SSD – How to Move macOS from HDD to SSD
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Besides the obvious issue of you having too much information to load onto the SSD, I've done this basic procedure in the past. However, it's much easier now using Time Machine, but you have a few options:
Time Machine
If you don't currently have a backup that can hold all your data, do yourself a favor and pick up a drive that can. Time Machine should be built into the OS you're currently running.
Do the backup. For 400 GB, you probably want to set it up before you go to sleep and let it run overnight. Once it's complete, unmount the drive and unplug it from the computer. Go through with the drive install and then install Lion. When the computer reboots, before you start to work with the setup assistant, plug in the new drive. At some point it will prompt you for a location from which to migrate data. Select your backup. Let it run for a while and when you get back, it will contain all the data you backed up, in a usable form. This is my current favorite method for cloning user workstations.
Super Duper
If you can't get an external backup, you can use SuperDuper to create a full-system backup from the active system disk. You'll have the ability to trim the data to the size of the disk to which you're copying in the interface. This route is a little more technical and requires a bit more time hands-on.
File Copy
I wouldn't recommend copying a user folder for a logged-in user due to open preferences files. You'll get all kinds of "File In Use" errors and it will probably exit the copy prematurely. Instead (if you're dead-set on using this method - which I advise against) create a second admin user and copy the folder over into a temporary location before staging it into the actual /Users/ location. You'll also need to make sure the permissions for the entire folder are set so that the user whose home you are copying can actually access and write to their files. To do this, you can use the Finder's Get Info on the folder and then add the user to have full permissions. Once it's staged into the /Users/ folder, you can log back in to the main user and delete the secondary admin. You may have permissions issues in the future using this method.
All that said, it's totally worth it to get an external to back up to for Time Machine to clone your new SSD from.
The Fusion drive is an amalgam of the SSD and HDD. If either piece gets removed or fails, you lose all the data. It might be possible to retrieve some of it, but it would be a job for drive recovery experts.
If you have a Fusion drive, it is possible to split it into the two separate pieces, resulting in a standard two-drive setup. However this is a destructive process, you need to reinstall the OS and all your data afterwards, as well as the recovery partition. On Macs that came with a pre-installed Fusion drive, you'll also run into issues where Disk Utility wants to "fix" the Fusion drive, so disk management needs to be done with the command line tools.
If you really want to split your Fusion drive, make sure you have a backup — this will delete your data; then boot into the Recovery partition (⌘R on boot) and open Terminal.
Run diskutil coreStorage list, and note the long string that appears after Logical Volume Group — that's the UUID. Then run diskutil coreStorage delete UUID, replacing UUID with the actual string. You can then reinstall OS X on the SSD or HDD and restore from a backup.
Best Answer
No - that won't be easy at all but it's exactly what Fusion Drive was engineered to do.
What would be easy is to make a backup of all the files you care about - preferably using Time Machine and a second clone copy if you're not sure Time Machine will work for all your files as a just in case.
Then delete about 30 GB of space so you have room to get a clean installer and make a bootable install drive with the OS of your choice.
When you boot from that installer - the trick will be wiping both the SSD and HDD entirely - then getting it back into Fusion Drive configuration.
Now that you have an OS and user data on Fusion arrangement - loading from your backup (either in the Migration Assistant portion of the new OS setup or running Migration Assistant) would get you the SSD boost without having to implement your own dual tier storage system.
Since most of these specific tasks are already asked / answered here and would be far too long for one answer - here's the outline and I'll try to link to detailed questions on each: