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A file in a directory has a Group ID "501"
After checking /etc/group, Group ID "501" was not found
how is that possible ?

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Perhaps a group with that ID existed at some point but was since deleted?

It's also entirely possible to, as root, chgrp a file or directory to a group ID that doesn't match any corresponding group.

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Did the file originate on a different machine? You often see this happening, for example, with unpacked tar archives that were created elsewhere. The user/group pair existed on the remote site, but possibly aren't defined locally. Therefore, utilities that try to resolve user/group numbers to names fail, and fall back to displaying the numerical ID instead.

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