It's most likely an emulation problem for backwards compatibility.
4k emulation sata usb controllers
Why is my USB drive showing corrupted data when plugged as an internal SATA drive?
I have a 3TB Seagate Backup Plus Desktop USB 3.0 drive, which works
fine when in its enclosure, but when I get it off its enclosure and
directly plug it as an internal SATA drive, it's just not properly
recognized (it works again when used in the enclosure). My systems’s
motherboard is an ASUS P8P67 LE, which has two SATA 6.0 Gb/s and four
SATA 3.0 Gb/s ports.
When used as an internal SATA drive Windows 7 asks if I want to format
the drive, as if it didn’t have a proper filesystem, and if I use the
DISKMGMT.MSC tool (typing that in the start menu) I get completely
wrong information about the drive.
It says the drive has 3 partitions (349,31GB of RAW data and two
unassigned partitions of 1698,68GB and 746,52GB). This information is
plain wrong, since the drive, when used in the USB enclosure, works as
a single NTFS partition (2794,52GB). How come it’s shown as 3
partitions without filesystem when connected through SATA?!
Is Seagate using some proprietary way of storing the data when using
the drive in its provided USB 3.0 enclosure? I didn’t use any kind of
encryption. I just plugged the drive with USB 3.0 and started using it
straight away, since it apparently came pre-formatted with an NTFS
See the accepted answer.
If you were to remove it from the case you would most likely find an adapter connected to it. Something like this. I did find one on an older 2 TB model. https://duckduckgo.com/?q=JMS578&t=ffsb&atb=v1-1&iax=images&ia=images