[safeharbourio]

well, with some experience, border guards will learn to spot a non-ntfs partition, that windows dutyfully reports as unformated/blablah space, so that will eventually be caught, dont rely on this to actually protect you, its more of a sleigh of hand that may be easily spotted by the right guard.

[netsharc]

Time to write one's own filesystem driver, that either hides this info or shows the disk as fully partitioned but empty NTFS partition, and when something tries to write onto it, throws a failure (so you can have e.g. 10 MB at the very front of the partition free, and the rest looks empty, but actually contains your Linux system)

Another hack would be one's own BIOS, that lies to Windows saying "This disk is 100 GB", but given the correct unlock signal, will admit to the OS "this disk is 500 GB big".

[lloeki]

What about dumping the partition table before simply removing the partition that matters? Just restore the partition table later. You could even grow the remaining partition without growing the actual filesystem, and hack something so that the FS layer reports the whole size. Or have both GPT and MBR that differ, offering two different views of the disk. If you're using lvm or btrfs you could also make a snapshot before removing all data then revert to the snapshot, and/or apply clever subvolume tricks like btrfs-convert does to keep the ext filesystem around [0].

[0]: https://btrfs.wiki.kernel.org/index.php/Conversion_from_Ext3

[r3bl]

Doesn't really work if your operating system doesn't support the partition format (such as ext3/ext4). From my experience, the only way this works is with TrueCrypt/VeraCrypt-encrypted non-system partitions.