[Samsung Galaxy Tab A series Questions & Answers] Nightsail:
THE SHORT VERSION: SM-T280 tablet keeps going into Download mode, even when I try to turn it OFF from the OS's power menu. I can get into TWRP. I can even get back into the OS. but every time I reboot, I have to jump through hoops, and the tablet won't stay off. After messing with its ROM, this problem seems unrelated to the OS it's running.

THE MORE DETAILED LONG VERSION:

I'm using an SM-T280 (Tab A, 2016 wifi-only model), successfully installed the LineageOS14 custom rom from this thread:

[ROM][LineageOS-14.1] Samsung Galaxy Tab A 7.0 - SM-T280 (gtexswifi)

Hello all! I've got the ROM working. Only problem is that once I add a google account and restart, I start to get the "Custom Binary blocked by FRP lock" message. Any way to disable FRP before the first reboot? Or force FRP to allow the custom...

xdaforums.com

I followed the directions and it works great! the T280 became my primary device for biking around with while using VLC for music and checking Maps now and then, and watching Youtube at home, even playing some little games. Sometimes the bluetooth audio acts funny, but I reboot the tablet and the problem is solved. I suspect the problem is actually my headset, but whatever; rebooting solves it for a few days.

And then one day, having done nothing new with it, I see that the power percentage won't go over 29%. It's been on the charger, and won't go up. Tried a different device on the charger; problem was not the charger. So I unplugged it, watched Youtube for a few, power percentage didn't go down.... so I shrugged and took it biking with me anyway. I figured maybe it was mis-reading the battery and would fix itself later. (It did; it eventually got down to 15% by the end of the day, so I put it back on the charger. It reported going back up to 100%) I'm not sure if the power-reporting issue has anything to do with the Download mode issues, but I don't know what might be a clue here.

But all good, right? I took it out to bike with again, and haha, no, the audio's being funny. So I figured hey, no big deal, just reboot it again.

Except it boots into Download mode, going straight to "Downloading... Do not turn off target". Almost all key combinations to reboot it again put it right back to this screen. Holding Power+VolDown+Home brings it back to the Download mode, but to the option to continue or abort. Aborting lets me then hold Power+VolUp+Home and get into TWRP. And from TWRP, I can boot into the OS normally. OS works fine from there - but any attempts to turn the device off or reboot it simply return to the Download Mode problem.

I've since attempted to restore it to stock ROM. I even tried telling Odin to do a Nand Erase while it re-flashed the stock ROM. I've reinstalled LineageOS. Recently reinstalled stock ROM while telling Odin to do Nand Erase, then made a TWRP backup, then installed Lineage and OpenGApps, then did another backup. It's currently waiting for Lineage to do its first boot; I'm watching the logo's circles slide across the arch....

Anyone know how to fix this?

Update: LineageOS didn't want to start this time, so I went back to TWRP and restored the backup I'd made of the vanilla stock rom.

Now I can get it into either Download Mode or TWRP, but Download Mode no longer wants to connect to my PC, and TWRP is doing this thing where it just runs a script or something automatically, then reboots again, rinse and repeat. I'm very confused.

What TWRP shows as it works (I took a video with another droid):

Could not mount /data and unable to find footer.

Failed to mount '/data' (Invalid argument)

Unable to recreate /data/media folder.

Updating partition details...

Failed to mount '/data' (Invalid argument)

...done

Unable to mount storage

Failed to mount '/data' (Invalid argument)

wipe_data_via_recovery

Full SELinux support is present.

Unable to mount /data/medium/TWRP/.twrps

Running Recovery Commands

Failed to mount '/data' (Invalid argument)

Formatting Cache using make_ext4fs...

Done processing script file

Failed to mount '/data' (Invalid argument)

Failed to mount '/data' (Invalid argument)

So I'm scratching my head again at this point. Thoughts?

(In case it helps: I have a Win 10 laptop as my primary device, but I do also have an Ubuntu 22.04 laptop someone gave me. I'm pretty unfamiliar with it, though. I was able to follow instructions to apparently install the Heimdall cli packages and copy the T280's TWRP, stock rom, and LineageOS files into its documents folder, but I really don't know what I'm doing with it.... and I'm reading something about an "EDL" cable? I can buy one, but might it even help in this situation?)