After an SD card is erased, what value is read from the card? All bits clear or all bits set?
The Official Specs from the SD Association should be your first port of call:
It is desirable to erase many write blocks simultaneously in order to enhance the data throughput.
Identification of these write blocks is accomplished with the ERASE_WR_BLK_START (CMD32), ERASE_WR_BLK_END (CMD33) commands.
The host should adhere to the following command sequence: ERASE_WR_BLK_START, ERASE_WR_BLK_END and ERASE (CMD38).
If an erase (CMD38) or address setting (CMD32, 33) command is received out of sequence, the card shall set the ERASE_SEQ_ERROR bit in the status register and reset the whole sequence.
If an out of sequence command (except SEND_STATUS) is received, the card shall set the ERASE_RESET status bit in the status register, reset the erase sequence and execute the last command.
If the erase range includes write protected sectors, they shall be left intact and only the non-protected sectors shall be erased. The WP_ERASE_SKIP status bit in the status register shall be set.
The address field in the address setting commands is a write block address in byte units. The card will ignore all LSB's below the WRITE_BL_LEN (see CSD) size.
As described above for block write, the card will indicate that an erase is in progress by holding DAT0 low. The actual erase time may be quite long, and the host may issue CMD7 to deselect the card or perform card disconnection, as described in the Block Write section, above.
The data at the card after an erase operation is either '0' or '1', depends on the card vendor.
The SCR register bit DATA_STAT_AFTER_ERASE (bit 55) defines whether it is '0' or '1'.
A file system really only checks the header of a file, sees the header says it blank and moves on. The OS then sees that as space that can be used, but until those sectors / blocks are over written, the data is still there and easily recoverable. If you want to see it, download the free Recuva, delete some files and then browse that directory with Recuva. It should scare you.