Did you format your data ("Format Data" with "Yes")? This step is necessary to access to internal storage. Look at the following pictures:
* Merge AOSP 9.0 r3 (Dees_Troy)
* Use ANDROID_ROOT variable instead of hard coding to /system (CaptainThrowback)
* Decrypt FBE on 9.0 and metadata decrypt (Dees_Troy)
* vold decrypt updates (CaptainThrowback and nijel8)
* Support vibration on LED class devices (notsyncing)
* Metadata decrypt support for Pixel 3 (Dees_Troy)
* Support rotating the display via build flag (vladimiroltean)
* Reboot to EDL mode button (mauronofrio)
* Support MTP on FFS devices (bigbiff)
* Update FDE decrypt to support keymaster 3 and 4 (Dees_Troy)
* Detect mkfs.f2fs version to properly format on f2fs partitions (Dees_Troy)
* Allow TWRP to use md5 and sha256 checksums for zip installs (bigbiff)
* TWRP can use /data/cache/recovery and /persist/cache/recovery on AB devices with no cache partition (bigbiff)
* Switch part of advanced menus in TWRP to use a listbox of options (Dees_Troy)
* Use magiskboot to allow repacking boot images for installing TWRP (Dees_Troy with thanks to topjohnwu of course)
I've installed the latest TWRP on my T580 and installed 4 different Pie ROMs. I backed up each ROM in TWRP but when I try to restore any of the backups, it semi bricks the tablet. The only way to get it working again is to use Odin to install a stock ROM and then start over installing a custom ROM.
That is actually the first one I tried and then tried 3.3.1. But I found the problem. I was actually backing up too much. When I left "System Image" unchecked, the backup restored OK.
I use adoptable storage and I have several problems with wipes and with the restore of partitions: internal storage and adoptable storage. When I try to do internal storage wipe and sd adoptable storage card, an error comes out. Same thing when restoring the backup. I can send some photos if necessary.
Can I ask you which rom you use on the t580 tab?
This may work for you.
Here you go, all mount issues should be fixed now.
BOOT IMAGE PATCHER
Boot image patcher to remove DM-verity and Forced encryption.
Flash the zip below with TWRP.
This was built by jcadduono not me, so props to him, I just modified it to work for this device and to remove the SEANDROID ENFORCING message.
NOTE: IF YOU INTEND TO ROOT THEN FLASH THIS AFTER SUPERSU.