Have a look at this debian forum thread
It has been confusing me, but apparently (with ext2/ext3 partitions), the ownership of the mounted filesystem is determined by the owner of the root of the partition being mounted, so (once the new partition is mounted), set the ownership of the mountpoint as appropriate, i.e.:
sudo chown <username>:<group> <mountpoint>
By changing the ownership of the mountpoint (/home/rob/Shared) after
mounting the partition, I managed to write to the the root partition of an old installation as an unprivileged user. This should only need to be done once.
Edited to add:
Just verified that - I unmounted the partition, set the ownership of the mountpoint to root:root
, then remounted, and ls -ld /home/rob/Shared
showed the mountpoint to be owned by rob:rob (and I could write to it again).
Well this is not even fun anymore.
Aah, the frustration of struggling with an obstinate computer system. Just think how good it will feel in the morning, though.