@PMD85Vette 

I'm on Windows 10 and I've never had a problem, and I've had many routers for a long time and going back to older versions of Windows.

I do have a CREDENTIAL to access the router though.

All the above are similar except for the IP Address or Network Name. 2 different IP Addresses, 3 NG routers, and 2 TP-Link routers.

All you need is either the Local Windows UID and P/W or the MS Account ones, depending on what you Windows uses. I'd assume the same for a MAC would be needed.

@PMD85Vette I sat at my computer this morning and just tried to write to the USB connected drive.  It magically worked!  Mind you, I did NOTHING to "fix" the problem.

I am currently mapped with router Admin account.  So I had some file maintenance to do, For about an hour, I was deleting and renaming files and folders.  All of a sudden I was no longer able to write to the drive.  Again, I did nothing in the way of configuring anything!  What is going on???? 

@PMD85Vette,

Good news, but it may not last.

You might be hitting the problem I occasionally run into.

I've got a fairly simple LAN, 2 PC's, 1 Printer, 2 Amazon Echo's, a FireTV stick, Smart TV, 2 iPad's, an iPhone, an Android phone, and an IOT device. Of course the router with a shared USB drive as well. The 2 PC's share the hard drives on them too.

Well, Router's seem to use NetBios to talk to the LAN and share the USB drive. Windows PC's use WSD. Different discovery methods, but Windows can handle it.

Now in W10, MS had some problems in V1709 (Creator Edition) and it seems it isn't fully resolved. That means the Master Browser doesn't always get the list of the devices or hand it off properly I think. The router, sincee its always on, we become the Master Browser and keep the list once all PC's are off. When a PC turns on, it becomes the Master Browser (by default) but the router doesn't always give it the device list... and that causes Windows sharing problems.

The prood of this is the CMD line NET VIEW command. Open a CMD prompt (CTRL+R at the same time and enter CMD). Then enter NET VIEW.

When working, it should look like this:

Only ones actually sharing devices, 2 PC's, Printer (USB ports), and TP-Share which is the USB drive on the router.

When it isn't, you'll get a NET VIEW error, AND you can't write to the USB drive. You can read it though DEPENDING on how you access it. Via a program, SAMBA is the access method and that will work. Used as a NETWORK SHARE, once you lose the LAN you can't access it at all.

Next time it doesn't work, open the CMD prompt and try NET VIEW...

Might want to read what I ran into similar to this, some days I can, some days I can't.... https://community.tp-link.com/us/home/forum/topic/249694?replyId=557156

@IrvSp Thanks for the info. My LAN is pretty simple (excluding 30 wifi devices). I just have a laptop and a network printer with the router/HD. I typically don't reboot the laptop because I have Plex server and TiVo server running on it. I lost the access again out of the blue today and was able to get it back by going into the router and dismounting the HD and remounting.  Hopefully this will be a good workaround that always works. It isn't like I need to write to the drive daily, so I guess I can put up with a workaround.  Thanks again, I'll check out the article tomorrow. 

@PMD85Vette,

I found that sometimes the unplugging and plugging back in the drive did help as well. Also just doing the 'Remove' and then 'Scan' worked too. However, only sure way was to reboot the router.

I suspect that forces a new list of devices to be made,

@IrvSp Thanks, that's what I did. Was sitting on the couch and didn't know the terms remove and scan.