There are some posts on this forum that discuss the details.
What I've found is that the USB 3.0 port understands USB 3.0 and the USB 2.0 port understands USB 1 & 2 and never the twain shall meet. YMMV. There is also a setting in the WiFi area that seems to turn the USB 3.0 port on and off. After you get the HD connected, you have to select a "USB Application" to apply to that port. I have a 3 TB USB 3.0 HD attached as both a SMB file share and as a DLNA media server. My Roku box sometimes has difficulty finding the DLNA server and doesn't always see newly added media files. The USB 2.0 port has a HP printer attached.
I am still not able to access my external hard drive when connected to my CellSpot Router. I contacted ASUS technical support. The ASUS tech support guy logged onto my router and said the router and hard drive were set up correctly. He said T-Mobile needs to send out a firmware update to enable accessing of external hard drives.
Evidently ASUS has its iCloud that fulfills this purpose, but T-Mobile crippled the router so this function is not present on the TM-AC1900. I would think AiDisk would serve this purpose, but I set up and log into the LAN FTP link and the Internet FTP Link and just get blank screens.
I called T-Mobile technical support to tell them what the ASUS tech told me. The T-Mobile tech support person just sent me the directions on how to upgrade the firmware. She had nothing to say about whether this router is even able to accommodate an external hard drive and whether a firmware upgrade would solve the problem. Once she found out my issue did not involve wi-fi calling, she seemed pretty eager to get off the phone. It does appear that I am running the latest firmware (18.104.22.168.376_3108).
This issue seems to have fallen through the cracks from a technical support standpoint. ASUS is not able to provide technical support for T-Mobile's modifications to the router. T-Mobile tech support indicated to me they are not able to support me on this issue since it does not directly involve wi-fi calling. It does, however, affect whether I decide to stay with T-Mobile as a customer. Obviously the "free" router was a huge perk, but that perk appears to have been illusory. At this point, I have spent so much time troubleshooting this router and have still not resolved the issue that I question whether it was worth it to get it for "free". I guess you get what you pay for.
As an additional step for debugging the problem,
Insert a USB stick, a brand new one fresh out of Walmart shelves, into the USB3.0 port on the router and see if you can access the stick from your laptop.
Because the problem could be the firmware on the external hard drive.
Why a brand new USB stick?
To ensure USB3 compatibility.
One thing I'll add re my TM-AC1900 is that when I write to the hard drive using a program that accesses the drive slowly over a period of 30-40 minutes, something fails and the file becomes truncated. If I write the file to a local drive then move it to the router, it works fine. I have no idea why. So, it's not exactly good as a general purpose storage server. I use mine mostly as a media server device.