Most likely the TC for myself. I'm trading my 4th Gen AE to my brother for his 16G iPad mini (I've done a LOT of free work for him and gave my many-a-parts, don't say I'm ripping him off. 😛) and I plan to move my 2TB TC to the modem and use it. Once I can get the $$ up, I'm planning on buying the 2TB and then add a 4TB to it. The 2TB will go into my MacPro.
It is a fairly delicate operation to do the HDD change over. I have a couple tips.
1. Find a very thin plastic, but stiff, spudger to get the base of. Two or more would help. The black base is a soft plastic and is marked easy.
2. You need a torx kit. 8 from memory.
3. Connectors are very delicate. I used long tweezers as I have used on minis and MacBooks in the past.
4. I left the sata cable attached to the logic board and only levered it off the HDD.
5. HDD is a snug fit. I removed it by holding the device end gently letting the HDD drop out the bottom, being super careful it does not grab wiring.
5. The HDD is only held in with rubber. You may need to reposition the rubber up inside the unit with say a driver before you fit the drive back in. It is very snug.
6. Upon rebooting the TC, you rebuild the HDD disk image through the airport app disk tab by choosing erase. You can rename it then.
Also, see ifixit for basic instructions.
See this thread if you want more details:
https://forums.macrumors.com/threads/1600698/