The 2TB HDD in my 2011 iMac has been spinning for 5 years now and I am very aware that the chances of failure are increasing. Since some months I am booting from an external SSD via Thunderbolt which works great. The HDD was 99% filled with pictures in Aperture libraries (which were back up-ed on external drives)- and given my concerns on the longevity of both Aperture and the HDD, I decided to reorganize and move everything to an external disk. The internal HDD is now intended for files of current projects, to be moved when finished. Going forward I have 2 question: 1) how can I remove OS-X and anything related to that from the internal HDD? Mac OS updates sometimes get confused by the 2 volumes and whilst updating the SSD, the iMac boots on the HDD (despite the boot disk setting). So I prefer to get rid of anything like that. 2) I was told low level formatting of a disk would re-vitalize it i.e. restore the magnetic capability. Which again I am told would make the disk faster to read but also reduce the risk of read-errors piling. Again I am being told, so would love to hear your opinion on this. I can also imagine putting an aging disk through a lot of cycles necessary to deep-format, mechanical failure is a risk.