Ubuntu: Disk Permissions & Multiple User Profiles
This is probably a no-brainer for advanced Linux users, but for newbies it may strike as awkaward that if you create/partition a disk with one user on your computer, another user won't automatically have "access" to it. This is a good opportunity to revisit the function of permissions in Linux--a quick review of Users and Groups.
TL;DR
When creating a partition, the "Disks" application/utility sets permissions to 700 by default (read/write/execute access for the disk owner only). You'll want to change that if you've got multiple users who need to access the disk on your computer. I'll discuss solutions below.