There you are, @Anonymous, as requested:

Before upgrading my 49XD8099 to Android 7.0 I had this scenario:

• Android 6.0.1 could not format my Lexar JumpDrive P20 64GB USB 3.0 pen drive as extended memory
• I could correctly use an old 60GB HDD as extended memory
• I migrated the "/sdcard" into the external disk
• A couple of weeks ago it became Read-Only and so it stayed.

After upgrading to 7.0 I tested what was the situation:

• The /sdcard was still mounted on external the HDD and still Read-Only
• A couple of apps in the HDD that disappeared showed up again
• Trying to move an app from the HDD to the internal memory Settings kept crashing (always)

So I did a factory reset.. And this became the situation:

• The Lexar JumpDrive P20 64GB USB 3.0 pen drive could finally format as extended memory, but taking a loong time (a dozen minutes?)
• I could successfully migrate the internal memory into the pen drive
• I could successfully copy files into the /sdcard (which was in the pen drive)
• Trying to move apps around Settings was crashing again

All was fine until few hours later, when the /sdcard became Read-Only again! i could not copy any more files (58GB free). The Memory Setting was nearly untouchable because it was a crash after another. The only USB operation I could do from the tv  has been to format the pen drive as external and that (I think) restored the internal memory as internal memory (losing the content but there wasn't much).

Oh, I also tried to use the old HDD again, but the crashes when clicking on any app location were constant again so I gave up. I didn't touch it since, to avoid another headache.