Wipe iMac HD through SATA to USB



I recently swapped my original HD in a late 2012 iMac for a SSD.  Everything worked well using a Time Machine back up.  The one thing I should have done is wipe the original HD before I removed it.  I'm attempting to do that now through a SATA to USB cable and am running into a few issues.


The original HD was partitioned with about 900gb for OS High Sierra and about 100gb for Bootcamp.  When I plug in the drive with SATA to USB, I don't see anything come up under finder.  With Disk Utility, I see another one that says diskOs4 (not the name of my OS partition) grayed out and unmounted and it won't mount if I attempt to.


Is there something I'm missing here or going about this the wrong way?


howapple - hello


Startup in macOS Recovery, select Disk Utility, highlight the Drive, not the indented partition(s), select Partition and Re-format the Drive. That will wipe and return the Drive back to a single 1T partition.