Now after a great deal of help from reading answers in this forum, I have my PC in perfect condition, bug free and up to speed. I now want to clear out my external backup drive and put a fresh clean clone on it, will a one pass cleaner be enough or should I go for the multipass option?
When you clone part of the procedure formats the drive and you lose all previous data.
A clone is an exact working copy that replaces the original C:\ HDD. You physically disconnect the cables of one HDD and put the other HDD in in its place.
I am not sure that cloning your C:\ to an external HDD will give you a working bootable new C:\ unless it is just a HDD in an enclosure and you intend taking it out of the enclosure and putting HDD inside pc. You would be better just to store a full backup image file of C:\ on the external.
Read your other thread. Good choice to make image file to external and not clone to it. You will be able to update the image or make new full backup image. It also means the rest of external drive will still be available for other files.
I have not seen your other thread, but assuming you are using Acronis - I would advise you to discontinue making Incremental backups, and instead make Differential backups.
Unlike Incremental backup, when all your daily backups creates the next file in a 'chain' of backups, a Differential backup creates an independent file containing ALL changes since the initial full archive. A Differential backup will restore faster than an Incremental one, as it does not have to pass through a long 'chain' of previous backups.
Also if any one of the 'chain' of Incremental backups becomes corrupted, the whole chain is rendered useless.
However, my personal preference is to make complete Full Backups on a weekly basis, doing away with Incrementals and Differentials.