Welcome to Security Identifiers (SIDs)
in the NT File System (NTFS)
.Long story short:
NT protects you from corrupting the file system by ensuring the running NT instance can reference every SID on the NT File System (NTFS). If not, it prevents you from access it. By 'taking ownership,' you are resetting the SIDs.Longer story ...
SIDs compromise of two (2) parts, one being an object reference (e.g., User Principal, Security Group, Domain Local Group, etc...) and the other being the "Computer or Domain" (e.g., local computer/registry or domain).
By taking ownership, you are applying a new set of SIDs to those files, solving the issue of the NT OS being unable to reference the SIDs.
E.g., your old Windows install that used the disk has its own, local computer/registry SIDs, which the new system you moved it to doesn't know about.