Yes, with SSD's it is trickier to get rid of a file even if you empty your recycle bin and then do a single or multi pass secure overwrite of everything that is in the the free space. With a traditional HDD, if you want to securely erase a file you shouldn't actually delete it by emptying the recycle bin, as that doesn't really delete the file. It is still there, can still be undeleted, unless it is meanwhile overwritten by another file or program. The fastest way to securely delete it without securely erasing the HDD's free space is to install a secure deletion software that places a context menu option so you can do it right away when you want to delete a file.
This whole process however doesn't work on an SSD. Why ? Because, unlike with an HDD where the HDD executes your overwrite command as you instructed, the SSD has a mind of its own in the form of the SSD controller. What it does is sometimes not at all what you told it to do but what it thinks is best for the wear leveling of the cells in the SSD. So, when you instruct the OS to overwrite the data with random 1's and 0's, your SSD will most likely put those 1's and 0's on other cells. So your apparently securely deleted file may actually only be overwritten the next time the SSD's controller has written to all the other cells once and comes back to these cells where your "deleted data" is.
So, the only apparent way to do it is by securely erasing the free space on the SSD from time to time, right ? Not exactly. When you do this you are actually wasting one cell cycle, so keep that in mind, but besides it may not solve the problem because the SSD has a spare area that it manages at its own discretion, and your deleted cells may no longer be worthy to be written to again (but can still be read from multiple times), so in fact you may be erasing a brand new cell that the SSD activated from the spare area, while your deleted data is still on the SSD on cells that can't be written to anymore.
The best way to securely erase an SSD is to actually destroy it. Of course in 99,9% of cases you will be fine with a secure erase procedure of the whole SSD because it's unlikely that you can actually recover full files from cells that have reached maximum write cycles, but it can happen. That's why government agencies have varying levels of secure data deletion, for both traditional HDD's and SSD's and other storage media like USB pen drives and memory cards. The devices that contained top secret information are probably always destroyed in the end, you can't get more secure than that.
Edited by tpi2007 - 3/13/12 at 3:42am