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I'm trying to share a single external hard-drive between two ubuntu different machines. The problem is, my user uid and gids are different across the two machines.

Is there a mount option for ext4 or ext3 that allows you to temporarily change your uid or gid?

Is my only option to have uniform uids across all my computers?

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mount can't change UIDs on file systems that support unix-style file permissions, like ext3 and ext4.

It seems you can use bindfs (package bindfs on Ubuntu) to do that (but I never used it). It requires you to mount your external hard-drive a second time and will likely reduce performance.

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binfs looks great, thanks I'll check it out – user61891 Dec 1 '13 at 16:02

If you have root access to at least one of the machines, you can add or modify the uids and gids to make them match.

The simplest thing would be to ensure both default gids exist on both machines, and your uid is in both groups.

Example with an overly simple passwd and group listing.

Machine A:
userA: 1003: group_defaultA
group_defaultA: 1002: userA
group_defaultB: 1003: userA

Machine B: 
userB: 1005: group_defaultB
group_defaultB: 1003: userB
group_defaultA: 1002: userB
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