HDD of Fusion has Bad sectors and I want to replace it. How can I do that?

Aside from the issues discussed here, you asked "What I am concerned about is whether replacing only the HDD portion invalidates the Fusion configuration". There is a process by which a fusion drive which has lost its 'connection' between the two drives can be reinstated or fixed. That is discussed on the link below. So doing what you suggest can be done. I just happen to think that it is more trouble than it's worth. An external drive solution is cheap, easy and quick.

How to fix a split Fusion Drive - Apple Support

Welcome to the Apple Support Communities.

You can take the Mac to an Apple Store for service on Fusion Drive, Get Support, or, you can easily use an external SSD as your startup disk. For this route, see: How to Setup and Use an External SSD as y… - Apple Community.

Cheers,

Jack

Vino909 wrote:

Thank you for your reply. I have been through the OWC videos and the physical process is doable for me. What I am concerned about is whether replacing only the HDD portion invalidates the Fusion configuration. I have seen several posts by Apple that say it either cannot be done or is not recommended. Leave me to scratch my head and wonder if an HDD failed, are they suppose to trash the entire machine? If the HDD is replaced, are there additional steps required to get it back in business?

Please don’t open the iMac. Simply use an external SSD. It will be faster than a new hard drive or Fusion Drive, and setting one up has zero risk.

If you are wanting another hard drive since you need a lot of storage, use an external SSD as your startup disk, then store large files and libraries on a high-quality external drive, such as a SanDisk Professional G-DRIVE - Western Digital or a G-Technology G-DRIVE mobile USB-C Portable Hard Drive - Apple.

Jack

Vino909 wrote:

What if I clone the MacHD to a new SSD, and stop using the fusion drive (SSD and HDD) altogether?

As long as the SSD is an external SSD, that's what I've been suggesting.

However, I also suggest you closely follow these steps and use Migration Assistant instead of cloning: How to Setup and Use an External SSD as y… - Apple Community.

you are correct in stating that the operation in question is exceedingly difficult. it is not something most people should attempt, much less actually accomplish. if you want, you can go to OWC’s website where you will find instructional videos for your specific iMac. then you can determine for your self if it's something you could do. you may also learn why that would be so expensive to have it done for you.

Or, an easy solution would be to purchase an SSD external drive and install your OS on it to use as a faster startup drive. They are now cost effective, compact, easy to install with vendor provided USB cables and most of them do not require any external power. you may have to reformat it, though. I personally like LaCies drives, but there are plenty of good ones out there. Doing this is easy to do and solves your problem.

In my posting above I provided you with a link that addresses your question. You can replace part of the fusion drives and then rejoin them with the procedure provided therein. That brings a fusion drive back to normal functioning. If you install the external drive and it has an operating system on it that is used for the startup drive, then what is or is not done to the drives within the computer does not matter. I hope that answers your question.

Vino909 wrote:

Jack: Thank you for your insight. I agree that cracking open the iMac is the last resort. I have no plans to do that. My question was geared toward understanding the process of actually replacing the HDD portion of the Fusion drive while retaining its function. The Apple Store would be glad to execute this process, if I hand over my first born. Funny how the answers you get depend not so much on the way you ask the question, but about WHO you ask. I've opened iMacs before with no problems. Most people won't. The other unanswered issue is:, if you employ an external SSD as you suggested, you negate the Fusion process. In doing so, could you run into trouble with subsequent Updates that address Fusion problems/issues?

I am not sure I understand. You say you don't want to open the iMac, but the only way to still use a Fusion Drive would be to have the internal replaced. Fusion Drive cannot have an external drive "fused" to the internal.

So, unless you go to the Apple Store, you cannot keep your Fusion Drive setup unfortunately/

No, they only have to be part of an OS, if at all. I think that you are over-thinking this way too much. What has been recommended here is done all the time and it works. Some people use an external drive even when their HD has failed.

+1 to Ronasara