Agree with
@punkncat:
A storage drive attached to the home laptop and
not shared otherwise would prevent the work laptop from gaining access to personal files. Likely the simplest and easiest to implement.
However if you use a NAS on your home network then that provides additional levels of access protection. I.e., the work laptop would need to be granted home network access and subsequent access to the network storage drive. Plus granted further access to personal partitions/folders on the network drive as so desired.
You would have to be very deliberate to "accidently" access personal laptop files via the work laptop.
The greater concern then being to somehow accidently put personal files on the work laptop.
What could be done there is to configure default path names on the personal laptop so saved files only go to the targeted shared drive or some network NAS. Weakness being that updates and apps may change those filepaths.
I.e., default destinations (as mentioned) for specific file type get changed. Still unlikely that that would target outside of the host personal laptop.
Not without some deliberate attempt to do so. (I have had more problems with defaults to online storage....)
Depending on how much admin access you have to the work laptop you could use firewalls to block the two laptops from each other.
An advantage of a home NAS is that you could set up the currently used external harddrive as a network drive via an enclosure designed for such purposes. Plug the enclosure into a router port. Configure and done.
FYI (informational only, not a recommendation or endorsement):
https://www.amazon.com/Networkable-Enclosure-APP-Connect-Connection-CD2510/dp/B0C4YBZ2V8
No need to transport the external drive any more. Yet, if necessary, you could still do so.....