[Android Q&A, Help & Troubleshooting] IronTechmonkey:
I know using a thread it not necessarily the best way to ask a question but since the cause of this issue is unknown (and might just be SD corruption) and there are a few other threads where it might come up, this thread can be used as a reference.

The issue is: An SD card that rather suddenly became unwritable in every way and can no longer be mounted by Android (although it is detected) but on which all existing data was undamaged, can be viewed and copied via USB card reader to Windows or Linux PC. The drive behaves much as if it was write protected or as if the file system (particularly the FAT) was corrupted in a way that mimics digital SD write protection. Here are the details.

L5.1.1/CM12.1 December 2015, SuperSU 2.65, TWRP3.0.2,

- 64GB Sandisk Ultra SD, initially accepted as formatted or formatted upon first use to Fat32; Working fine for a few months as repository for static data (ROM zips and app apks), work material (frequently edited text, DOCX and XLSX files), as well as apps and app data with a dynamic presence on the drive.

- Drive suddenly stopped appearing on device in CM and in TWRP. When re-inserted in CM, the system detects a card but times out as it is unable to find a mount point. Attempts to mount SD card in ROM result in the following error in CatLog:

"E/Vold (222): /dev/block/vold/179:65 failed to mount via VFAT (No such device or address)

E/Vold (222): Volume sdcard1 found no suitable devices for mounting :("

- Attempts to repair, partition or format the drive in TWRP result in mount error and the details of the drive have zeros where there should probably be other values.

/external_sd | | Size: 0MB Used: 0MB Free: 0MB Backup Size: 0MB

Flags: Can_Be_Mounted Can_Be_Wiped Wipe_Available_in_GUI Removable Is_Storage

Primary_Block_Device: /dev/block/mmcblk1p1

Alternate_Block_Device: /dev/block/mmcblk1

Display_Name: MicroSD Card

Storage_Name: MicroSD Card

Backup_Path: /external_sd

Backup_Name: external_sd

Backup_Display_Name: MicroSD Card

Storage_Path: /external_sd

Current_File_System: auto

Fstab_File_System: auto

Backup_Method: files

MTP_Storage_ID: 65537

- With SD in card reader attached to PC, the contents of SD are visible and can be copied to PC but files cannot be copied from PC to SD card. It looks like they are copied but they are gone when card reader is disconnected then reconnected. Same in Linux Ubuntu 14.x.

- Unable to delete or reformat partitions in any utility that I tested including: Dedicated GParted boot USB, EaseUS Partition Master, HP USB Disk Storage Format Tool on PC, GParted in Ubuntu 14 VM, GParted in Ubuntu 10.x live CD/USB.

- Although the SD drive shows as Fat32 in GParted, Windows 7 only offers exFat and NTFS as formatting options. Some research shows exFat can digitally "write protect" drives with a dirty bit or byte which causes very similar symptoms.

- Windows Chkdsk requests to convert lost chains to files but must not be doing so as this occurs on every scan.

Soooo...the questions are: What in the realm of relatively normal android use might cause this? Has anyone heard of a disk or partition management setting or common form of drive corruption that could cause this? Could such a thing be caused by a file manager? A 32GB replacement SD is doing OK but I'm left wondering if the issue with the 64GB SD card was caused by a utility on the device or if it was just SD corruption (that looks conspicuously like digital write protection).

[EDIT] A minor correction I should make: The reason Windows 7 only offers NTFS and exFAT is not because the drive is damaged (although it may be) but rather because Windows 7 cannot format USB drives over 32GB with FAT32 although it can mount and use larger FAT32 SD drives that were formatted elsewhere. In this case I let android do the initial format months ago (Fat32) and never had problems during the times I connected it to card reader (although I usually leave it in the device and use Wifi or MTP through device USB to PC).