I did a test migration last night (v2017 Premium to v2020 Premium, same server, both SQL instances are local), with the new instance's default data directory changed to the E-drive. When I installed v2020, the MAS_ABX and MAS_SYSTEM files were properly in the E-drive path.
However, when I ran the migration the company databases had their mdf / ldf located on the C-drive, and MAS_SYSTEM had it's files moved to the C-drive too.
Now, when I created a test company, the mdf / ldf are properly on the E-drive, so SQL is fine... just the migration program seems to cause a problem with the restored file locations. Is this a known issue?
I have moved the files, and can deal with this manually again when doing the Live upgrade, but in situations when C-drive space is not abundant this could seriously cause problems.