[Using 'Erase Free Space' on iMac with Fusion Drive]

9 hours ago, nukecad said:

I don't think that it's possible with a fusion drive.

Presumably you want to erase the drive because you are selling the Mac?

I think that your only option would be to split the fusion drive into a SSD and a HDD.
That can be done, (and they can be merged again), but that means reformatting the drives each as a seperate drive, so you would wipe everything on there (including the OS) not just the free space.
Of course you would need to re-install the OS again.

This recent (Feb 2019) article walks you through the process, read the comments as well:
https://www.tech-otaku.com/mac/secure-erasing-mac-fusion-drive/

Ok thank you for taking the time to reply.

[Using 'Erase Free Space' on iMac with Fusion Drive]

I have just downloaded CCleaner for Mac with the intention of using the 'erase free space' facility. However, when I try to do this I get a message in the Details box that states 'Erase free space is not available for SSD's'. I have a fusion drive, 128gb of which is the SSD. Is there any way I can wipe the remaining (non SSD) HDD using CCleaner? If not then I have a program that is of no real use to me!

Any advice would be gratefully received!