your thought/theory are unlikely
the most against your thinking was declared in the other thread (closed cause of "layer 8 problem" on both ends).
again:
mostly a filesystem gets corrupt when the drive is disconnected while being in use/mounted (e.g. hard power cut).
against the "boot theory" and for the "dear Pi don't boot my external drive":
usually LE is configured where to boot from during LE installation.
with the UUID's form the disk what is attached during installation.
in an LE terminal the command
dmesg|grep "Kernel command line:"
will show what UUID's are involved.
there are usually two entries (from my sata ssd):
- boot=UUID=...
and
- disk=UUID=...
what UUID belongs to what partition/disk is shown by the command "blkid"
long story short:
I'm not aware off by only attaching an external disk would change the above entries where to boot from !
- that changes need more work -
therefore I'm - for the main part - NOT convinced that the external disk is bootable at all and if it would somehow, with the lack of needed boot files you would see an error message only (unable to boot from ... or such ), but it wouldn't corrupt the filesystem.
===
to boot from a disk you additional need the disk marked as bootable (don't know if that is still true with EFI !)
if you only partitioned and formated your external disk this bootable flag is usually NOT set (unsure).
there are some tools (fdisk, parted, etc.) out there to see and change the boot flag.
!!! playing with those tools could damage partitions !!!
parted is available on LE.
please read the help files/manuals for parted, e.g. "man parted" => internet
there the "print" command and how to find out if a external disk IS bootable or NOT
my LE internal and bootable sata ssd:
...
1 4194kB 541MB 537MB fat16 System boot, legacy_boot, esp
...