Hi Ralph,
Thanks for your reply. No problem with the "late" reply, we all have our own commitments (and also live in different parts on the globe 😉).
First part of your answer: yes indeed, it does make sense. Thanks for offering to do the research about the different types of algorithms. However, I must say that on the iMac, both DU and iStatMenu (and CleanMyDrive) report exactly the same available/used SSD size.
So, I'm anxious to know the real size available i have on my MacMini SSD. I must say though the difference in size reported on the MacMini **might** be due to the fact that when I first migrated from the MBA SuperDuper Clone, the process failed, I got a black screen and had to turn off the Mini, turn it on again after waiting for awhile and launch the whole setup/migration process a second time (with success).
As for the second part:
1. Recovery partition problem: I did reboot pressing Option key but couldn't see the Recovery HD.
This might be due to the fact I'm using the iMac screen (the only one I have) as the default monitor for the MacMini. Right after rebooting the MacMini, I had to press cmd-F2 on the iMac's keyboard in order to have access to the MacMini monitor. So although, right after, I immediatly pressed the Option key on the MacMini's keyboard, the above mentioned manipulation (cmd-F2 on the iMac's keyboard) prevented the MacMini to be aware of the fact that I was actually trying boot on the Recovery partition...
Is there a workaround? (I tried several times, should I try again?).
2. I assume the very last paragraph of your reply refers only to a Clean Install (which I understand you don't recommend and which is in fact very good news for me). What you explain has nothing to do with a simple rebooting (on the unreachable Recovery partition) and reinstalling Mac OS X WITHOUT erasing my hard drive, if not please correct me.
Thanks again for your detailed explanations 🙂