MacOS – Mount a disk image as physical USB drive

Have you opened Disk Utility and tried to format it already? Sometimes a drive will not appear on the desktop, but will appear in Disk Utility.

I've had windows drives show up sometimes, and not other times. What you could do to make it more likely to show up is go to an available (hopefully not too difficult) windows machine and reformat the drive as a FAT/MS-DOS drive (don't do quick-format). This should be easier for the mac to see.

Once you can see the blank FAT formatted drive, open Disk Utility and then select it on the left sidebar and erase it again as a "Mac OS Extended (Journaled)" (HFS+) format.

There are other methods like using an emulation environment like Parallels Desktop (IMHO bad SW company), VMWare Fusion (my choice), or VirtualBox (free) (all of these require a windows install disc); or you could even try using programs like MacFuse or NTFS-3G, Tuxera, etc., to mount windows (NTFS) drives directly in the Finder. But unless a mac you can find has one of these solutions, it's probably not worth it to go through the trouble just to erase one drive.

Lastly, I've also had some external USB enclosures not show up on some versions of OS X! So the cheapy enclosure you got just might not work with your installation anyway?

Details that might be helpful for potential helpers:
HD interface PATA/ATA/IDE or SATA?
OS X Version?
iMac model year or identifier?
Enclosure is assumed to be USB 2.0 high speed plugged into built-in port on iMac. Correct if wrong.

The save button is checking all mounted drives, and will spin them up.

Your only way I can think of to do this is to prepare a couple of scripts.

See the man page for diskutil. If the mount point (diskname) is BigFatDisk

then to unmount the disk it would be something like

diskutil unmount BigFatDisk

Now to keep everything sync'd to BFD you would run a script now and then

diskutil mount BigFatDisk rsync flags local disk BFD diskutil unmount BigFatDisk

A second option is to replace the disks in the BFD enclosure with hybrid disks. These have an SSD chunk and a spinning rust chunk, and are ideally suited for this.

A third option is to let your disks spin whenever the machine is awake? The amount of power they use is small, and increasingly MTBF times are being given as X spinups, not X hours. A hard drive 2 years ago used 5-7 watts. I'm sure it's better now. So let's use 5. That works out to 40 kWhr per year, roughly. At 15 c per kWh that's 6 bucks/year. Seems to me that this is pretty cheap compared to the hassles you have now.

I have set my computer to turn the screens off after 20 minutes idle time, but never to turn off the CPU or spin the drives down.