How can I prevent auto-mount of 2nd internal drive? (Ubuntu-MATE 20.04.3)

Question

When I log in to my main Ubuntu-MATE 20.04.3 (nvme0) OS, my secondary OS install (/dev/sda) is somehow already auto-mounted on the desktop.

I'd like it not to be.

/dev/sda is an internal SSD on which I also have a bootable Ubuntu-MATE 20.04.3 installation.

In my top panel, I've placed the "Disk Mounter" applet, (put there by right-clicking on the top panel and choosing "Add to Panel." Disk Mounter is one of the many applets that users can put in the panel).

Via Disk Mounter, I have a disk-icon in the top panel that I can click on to mount another internal SSD that is for storage only.

Normally I should also have a second, identical-looking disk-icon there right beside it to click on that mounts the /dev/sda SSD.

In fact, I do have that second disk-icon for /dev/sda, but for some reason it's oversized -- larger (in fact, it's a different icon) than the one beside it, and it has the small lime-green triangle lit up below it, indicating that the /dev/sdadrive is already mounted (until I unmount it).

This is on a desktop workstation running (as indicated above) Ubuntu-MATE 20.04.3, kernel 5.4.0-96-generic.

Here is my /etc/fstab file:

(UUID 07041... is the main OS's ext4 partition, and UUID 2BD2... is the main OS's EFI partition)

  1 # /etc/fstab: static file system information.
  2 #
  3 # Use 'blkid' to print the universally unique identifier for a
  4 # device; this may be used with UUID= as a more robust way to name devices
  5 # that works even if disks are added and removed. See fstab(5).
  6 #
  7 # <file system>  <mount point>  <type>  <options>       <dump>  <pass>
  8 UUID=07041e0f-1cde-4caf-9b1d-86851e9601ed   /   ext4    errors=remount-ro   0   1
  9 /swapfile                                 none            swap    sw     0   0
 10 UUID=2BD2-4199   /boot/efi    vfat     defaults      0       1

And here is the output of:

--> lsblk -e 7 -o name,fstype,size,fsused,label,partlabel,mountpoint,uuid,partuuid                                  pts/2  Tuesday  2022-01-25  14:29:20 
NAME        FSTYPE   SIZE FSUSED LABEL         PARTLABEL                  MOUNTPOINT UUID                                 PARTUUID
sda                894.3G                                                                                                 
├─sda1      vfat   550.3M        SSD_ESP       ssd_EFI-system-partition              6DE5-0FF8                            684141f2-20c3-41be-b2c4-7058d9488896
└─sda2      ext4   893.7G        20.04.3-ssd   UbuntuMATE-20.04.3                    21697ef7-83e3-4414-a859-91841f61dce8 7cc6f822-eb23-4829-9039-6d09e6e0ca55
sdb                894.3G                                                                                                 
└─sdb1      ext4   894.3G        WeirdBeard    WeirdBeard                            70e15e24-a75c-4115-9c83-29cb585676f6 5ab70820-78e7-466e-9c0f-101054d5df9b
nvme0n1            931.5G                                                                                                 
├─nvme0n1p1 vfat     402M   9.7M NVME0_ESP     nvme0_EFI-system-partition /boot/efi  2BD2-4199                            337ce8d2-65a0-4051-8c12-55cc22666e7e
└─nvme0n1p2 ext4   931.1G    40G 20.04.3-nvme0 UbuntuMATE-20.04.3         /          07041e0f-1cde-4caf-9b1d-86851e9601ed 0657b873-53c2-4a40-9e60-3b26c891fd1b
nvme1n1            931.5G                                                                                                 
├─nvme1n1p1 vfat     402M        NVME1_ESP     nvme1_EFI-system-partition            E23B-29C2                            90af4214-eca8-4265-8e5f-9c2e3301bba9
└─nvme1n1p2 ext4   931.1G        EMPTY-nvme1   The-next-LTS-UbuntuMATE               a3cb2757-686f-4c5c-92ca-a43b3a223207 77706804-715d-4276-9824-a184ed00df4f

(sdb1 -- "WeirdBeard" -- is the storage drive; nvme1 is empty at the moment. They can both be ignored.)

(I tried the answer posted here (from 2014) -- it prevented the auto-mount, but this only made it so I couldn't mount /dev/sda at all.)

One problem that I'm not sure what to do about is that the /dev/sda drive, after a few minutes, reverts (on its own) to being mounted at /run/timeshift/backup (that is, if I don't leave it mounted on the desktop).

sudo timeshift --check shows:

/dev/sda2 is mounted at: /run/timeshift/backup, options: rw,relatime,stripe=8191

So: sda2 is always either

(a) mounted upon login at /media/<myusername>/20.04.3-ssd <-- (the label I gave this partition) until I un-mount it;

(b) mounted at /run/timeshift/backup; (during which time its icon does not appear in the top panel) or

(c) not mounted.

My question: How can I get the /dev/sda drive to NOT automount at login (but TO mount when I click its icon in the panel), and how can I get the top-panel icon uniform with, and same size as, the other disk-icon there?