how to retrieve data on SSD part of a fusion drive ?

I doubt that is possible, but I think your best bet is to ask Apple. They might have special tools for data recovery. It should go without saying that you should have had your data backed up. Never again leave yourself without backups.

 

The fusion drive is made of two components: one hard drive and one ssd blade. You can remove them from you imac and install them in another one.
But you have to install them together, data is split between the two components at block level (not file level).

 

No.
This simply isn't possible.

You can't get data off "half of a fusion drive".
Well, perhaps Apple can retrieve it, but an end user isn't going to be able to do it.

This is what "backups" are for.

The only way I can think of that could possibly do it:
1. Get another iMac that is similar to yours
2. Take BOTH the SSD and HHD from your iMac and put them into the second.
3. See if the drives will "mount" as a single fusion drive that way.