That disk already has over 2TiB of partitions on it. Ordinarily, that's not possible with the Master Boot Record (MBR) partitioning system; however, some tools do let you take advantage of quirks in the MBR definition and create up to 4TiB worth of partitions on such disks so long as the last partition begins before the 2TiB mark. Your disk satisfies this rule, so it's conceivable this is what you've got. To verify this, please examine the output of:
If you get a line that reads "Partition Table: gpt", then my hypothesis is incorrect and you should ignore the rest of this message; instead, post back with more details about what happens in GParted when you try to resize your partition.
If the line reads "Partition Table: msdos", then my hypothesis is correct. In this case, IMHO your best bet is to convert the disk from MBR to GPT format. You can use gdisk to do this, as described here.
(You'll probably have to install the gdisk package, which I believe is part of Mint.) The conversion process itself is extremely simple -- just launch gdisk, verify that you're working on the right disk by reviewing your partitions with the "p" command, and save the partition table with "w". The complication is that you'll then need to re-install your boot loader. Assuming you're using GRUB, this works best if you place a BIOS Boot Partition
on your disk, and it may need to go before
the 2TiB mark, so you may need to shrink one of your existing partitions to make room for it. The BIOS Boot Partition can be quite small -- often under 1MiB, although I've seen claims that some versions require up to 2MiB. Note that's MiB, not GiB. It's identified by a type code of EF02 in gdisk or by having a "bios_grub" flag set in parted or GParted, and it contains no filesystem.
All of this is best done from an emergency recovery disk -- either Mint in its "try it before installing" mode or something like System Rescue CD
or Parted Magic.
You could do the partition table conversion from your regular installation, but resizing your partitions would require unmounting them, and it just gets hairier from then on.
One more point: If your motherboard is new enough, it may support UEFI
booting, and converting from MBR to GPT might
make the motherboard favor that boot method. This is unlikely, but possible. If you run into this problem, you may need to create an EFI System Partition
rather than a BIOS Boot Partition and install an EFI-mode boot loader
rather than a BIOS boot loader. There are also a small number of BIOSes that have bugs that prevent booting from GPT disks.
Many such BIOSes are actually early UEFI implementations, though, so you may be able to switch to EFI boot mode with them. Most other such problems can be worked around, as described on the page to which I've just linked.