You likely have a few options. 15GB is a bit small for a desktop drive and you've likely run out of space on it. You could reinstall Elementary setting the HDD as your /home and /usr partitions. Another option would be copying everything over to the HDD, but this is likely to make boot slower. A third, more advanced option would be setting the HDD to be your /home and /usr and transferring everything over. They're essentially in order of how easy they are.

BEFORE DOING ANY OF THESE, ENSURE THAT YOU'VE BACKED UP ANYTHING YOU CARE ABOUT.

THEY WILL ALL WIPE THE HDD, THE FIRST WILL WIPE YOUR SSD AND BOTH THE FIRST AND THIRD WILL MAKE MAJOR CHANGES TO YOUR SSD's FILE SYSTEM THAT WILL MAKE IT IMPOSSIBLE TO BOOT ELEMENTARY WITHOUT BOTH THE SSD AND THE HDD PLUGGED IN.

Reinstall elementaryOS

This is the best solution if you don't really have anything on your system.

Backup everything, it will all be deleted. When installing, select "something else". Select "New Partition Table" for both drives (the /dev/sd{x} without a number). This will erase everything on your drive. On your HDD, create a new partition by clicking "free space" then "+". You can leave the defaults, except set your mount point as /home and size to about 425000 (about 425GB). Repeat step 4 on the HDD, set the mount point as /usr and let the size be the rest of disk. On the SSD set the mount point as / . Ensure that "Device for boot loader installation" is your SSD. Finish the installation.

Copying everything over

This solution will keep your files intact but will mean that your SSD won't be used for boot or anything else.

BACKUP EVERYTHING YOU CARE ABOUT OR ELSE. Boot into your elementaryOS install disk/USB key. Open the GParted Partition Editor. Identify the SSD and HDD's disk identifiers ( /dev/sd{a,b,c,etc.} ). Open a terminal and run sudo dd if=/dev/<source disk, the SSD> of=/dev/<dest. disk, the HDD> bs=4M ( dd copies the entire contents of the if file or disk to of , bs=4M tells it to copy 4MB at a time). Wait until it finishes, it won't show you your progress and will take some time (for 15GB to a HDD my guess is an hour or so at most). Remove your SSD from your computer and reboot.

Change your mountpoints

This solution is quite advanced and should work, but I haven't tested it. Use it at your own risk. You may lose data or end up being unable to boot. Read through the entire instructions. If you're not sure you can do everything below, don't try to.