I have a laptop in which my username is, say, "foo". I have an external drive, formatted with Ext4, for which all files are owned by "foo" (at a filesystem level). Now, I have a desktop in which my username is, say, "bar". If I mount this external drive in this computer the files are considered to not be owned by "bar". This makes sense, but it is annoying because their bits mode are set so that only the owner can modify/delete them. What's the cleanest way to deal with this? Create a group with "foo" and "bar" and add group modification permissions?
Ownership and permissions don't make much sense on media that moves between different machines with different sets of user accounts. Consider using a filesystem that doesn't have any ownership info, such as UDF or FAT32.
BTW, it's not the username that matters, it's the UID number. If the user "foo" on machine A has UID 1000, and the user "bar" on machine B has UID 1000, then machine A will see the files as owned by "foo", and machine B will see those same files as being owned by "bar". The number is what's actually stored on the disk.