2 SSD startup disks for iMac

your mac computer specifications:

iMac (Retina 4K, 21.5-inch, 2017) - Technical Specifications

it has two Thunderbolt 3 (TB3) ports (each port can utilize max 40 Gbps, aka 1600 MB/s aka 1.6GB/s).

b = bit . B = byte.

8 bits = 1 byte.

definitely you can add USB-C 3.1 Gent2 (max 10Gbps) based external drives , or TB3 (40Gbps) based external drives.

You can use TB3-to-M2/NVMe Storage Memory ENCLOSURE(s), such as:

OWC Envoy Express Thunderbolt 3 Enclosure for NVMe M.2 SSD, $79 :

https://www.amazon.com/dp/B08LYQPRPJ

( it can accept any PCIe NVMe M.2 M-Key 2280 SSD storage memory )

SABRENT Thunderbolt 3 Certified M.2 NVMe SSD Tool Free Solid Aluminum Enclosure (EC-T3NS), $99 :

https://www.amazon.com/dp/B08FT59SB6?th=1

( it can accept any PCIe NVMe M.2 M-Key 2280 SSD storage memory )

Then add your own capacity/size choice of SSD storage media stick that are M2 category : such as 1TB or 2TB or 4TB etc.

Even though these M2 type of drives can give very very fast data rate, but TB3 interface in the enclosure will allow only upto 1600 MB/s max data rate.

your current internal (fusion) drive is operating at max 6 Gbps (aka: max 600 MB/s) SATA-III based interface.

fusion-drive is a hybrid category drive ( which has smaller size faster-SSD, & larger size slow-HDD memory), so slow & faulty.

so TB3-to-M2 enclosure based external SSD storage drives will be nearly 3 to 6 times more faster , than SATA-III based fusion-drive.

replace the fusion drive with a full SSD drive , use disk-cloning software to copy from old fusion to new SSD.

must make "Time Machine" full backup, before that.

Ofcourse you can boot into any one out of multiple external drives, with multiple different version of macOS,

During boot, press T to select which boot-media you want to boot from.

Mac startup key combinations - Apple Support

Use more than one version of macOS on a Mac - Apple Support

You should see my earlier this , this post, for more on creating boot media.

exactly at this moment (Dec 2022), the Monterey is still much better than Ventura,

but your usage software, OS, your hardware etc will be different than others.

yes, you could use more than one external SSDs as separate boot drives, each with independent macOS. but you will likely have one issue. your internal drive is running Mojave. in order for you to install Monterey on an external drive, you will first need to install Monterey on the internal drive. the reason is that when installing Monterey, the Mac needs a firmware update that is applied when you upgrade. and once that is done, you will then be able to install Monterey on an external drive.

EDIT: the firmware update requires, and checks for, an apple OEM drive.

Attempting to Install Ventura or / and Monterey on a Non Apple Original drive ( be Internal or External Drive " is problematic at best.

That does not mean it is impossible but problematic

Would suggest referring to @jefferythefrog posting related the EFI Boot ROM aspect of that reply.

👍 👍

TY. :)

😀

You may find while booted into Mojave, that Mojave will pop up a notification regarding an issue with the Ventura drive. This is because macOS 10.15+ has a new drive layout which Mojave is not familiar with. I've seen Mojave complain about the "Update" volume when a Catalina boot drive is connected. This may not be a problem with Monterey or Ventura, since I think both of these operating systems have lost the "Update" name for the update volume, but I'm not certain since Mojave may still see other system related APFS volumes....just make sure not to have Mojave touch any system related volumes on a 10.15+ drive.