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#11
That's right if your current OS partition booting operation is corrupted then the Clone will be no good too; imaging it will only be good to recover folder/files of desire, but not to boot it.
The point of sucessful backup imaging is to image your OS Partition or Clone your HD Often, as usage dictates - that way when you really need it, you will have always have good working bootable backup. Naturally you will need to restore it to a good drive irregardless.
As for you last question quoted. There are softwares that can recover sectors that were marked bad and taken out of service, and can recover them and the data they contain. I've done it successfully many times, and other times you can not. Many times the heads of the HD physically break and in that case no matter what software you use can not recover - unless you have the ability to remove the platters of the old into a new identical case and continue, but this process is mostly left to the professional HD recover companies.
Otherwise, if it was just an anomaly caused sector to be marked bad and taken out of service, the program I've used to successfully restore that sector and it's contents to working status is a program called SpinRite.
If sectors are being marked bad often, or if other indicators call the drive into question, then it's time to replace the HD asap anyway no matter what.