@hellothere don't want to leave this unanswered.

Prereq:
Have a USB drive or SSD with same or larger capacity than your SD card.
When using an SSD SATA to USB adapter make sure it is fully UASP to get max. transfer/access speed. Here are some hints [1] - also this site has more details as I do provide below.

It goes like this:

Make a full backup of your SD-card (eg. with Win32DiskImager (<- easiest), Clonezilla, ...) to your desktop PC. Put that backup at your USB drive / SSD (again using Win32Diskimager or the tool of your choice) on your desktop PC. Resize the USB partition (only if the USB drive is larger than SD card). Advise the pi to use a different partition after initial booting from /boot, handing over to the USB drive.

When you are done with the backup (Step 1 and 2 from above) attach both (SD card and USB drive) to the Rpi.
Remove any additional USB storage attached to the pi to avoid confusion.
Boot/Power up.
Nothing has changed up to this point.

Note: Following steps are the crucial part - but no worries you have a backup. If something derails, you can back out by using the backup.

Step 3: Optional: Resize the USB partition (if USB drive is larger than SD card).

SSH to your pi as user pi. Type

sudo su

Issue blkid /dev/sda*

blkid /dev/sda* /dev/sda: PTUUID="e006ca92-3704-44b9-a15f-bc56e2171825" PTTYPE="gpt" /dev/sda1: UUID="4832bb70-bb77-4a4a-9d68-5541559df133" TYPE="ext4" PARTUUID="9ec38a7c-ff79-413f-bddc-fc66de260c13"

Note the partition number /dev/sdN on the left which has the TYPE="ext4".

Run with that number resize2fs /dev/sdN (replace N with your number identified, most likely it will be 2).
Wait for process to finish.

Step 4: Advise the kernel to mount a different partition as "root-filesystem".

I underlined the items to REMOVE in RED and those to ADD in GREEN.

The red and green values are different on your machine. Use those. Not those from the example!

SSH to your pi as user pi. Type

sudo su

gets you root.

Issue blkid /dev/sda*
B1.jpg

Will give you a similar output as above.
Note down the value of PARTUUID of partition with TYPE="ext4" (in the example 9ec38a7c-ff79-413f-bddc-fc66de260c13). There should be only one with this type.

Edit fstab (with editor nano (<-- easiest) (or editor of your choice)):

mount /dev/sda1 /mnt # <-- this should be the same sdN as used in resize2fs, will mount the partition from the USB drive. nano /mnt/etc/fstab

You will find something like this:

B2a.jpg

These current PARTUUID point to partitions on your SD card.( You can verify it with blkid /dev/mmcblk0*)

The second entry has to be altered to contain the PARTUUID you noted down before. For example:

B2b.jpg

Again: Use your identified PARTUUID value.
You don't have to keep the lines starting with # (=comment)
Save the file.

Almost done: Then edit cmdline.txt, last step before rebooting:

nano /boot/cmdline.txt

You will find something like:

B3a.jpg

This instructs the kernel to look for the root filesystem at the partition PARTUUID, currently the SD card.

Make it read, change only the PARTUUID, leave everything else as is:

B3b.jpg

Save the file. Reboot... Drum roll, please!

After reboot, ssh to your pi, then check with mount command:

mount | grep sda

Should output something like:

/dev/sda1 on / type ext4 (rw,noatime)

That's it. Revel in your success.

[1] https://jamesachambers.com/raspberry-pi-4-usb-boot-config-guide-for-ssd-flash-drives/

Can fan be connected to output