You can also keep your blockchain on multiple drives if you use Linux. So if you have another hard drive you can start downloading the blockchain on that hard drive and then change the data directory to the one on SSD and make soft links to the block files on the other drive. (soft links are like shortcuts)
You can make soft links by using a command "ln -s ", which will make a file that will point to the when accessed. In this scenario it makes sense for target to be on the regular hard drive while the soft link (shortcut file) to be on the SSD.
I use Windows 7. I know I can use NTFS junctions to point to another drive as well as choose different datadir in startup parameters or bitcoin.conf. The problem is that my main computer is now a laptop, not a desktop. Sure, my laptop have small 256GB SSD system drive and 1TB 7200RPM secondary mechanical hard drive, but today I think it is bare minimum for power users like me. For desktop it is possible to throw in just another multi terabyte drive, but for laptop the expansion is somewhat limited. You can make soft links by using a command "ln -s
That is to say, you'll need to redownload the entire blockchain again.