Electronic – SD card detect in SPI mode

That's what I've found:

The SD 1-bit protocol is a synchronous serial protocol with one data line, used for bulk data transfers, one clock line for synchronization, and one command line, used for sending command frames. The SD 1-bit protocol explicitly supports bus sharing. A simple single-master arbitration scheme allows multiple SD cards to share a single clock and DAT0 line. The SD 4-bit protocol is nearly identical to the SD 1-bit protocol. The main dierence is the bus width bulk data transfers occur over a 4-bit parallel bus instead of a single wire. With proper design, this has the potential to quadruple the throughput for bulk data transfers. Both the SD 1-bit and 4-bit protocols by default require CRC protection of bulk data transfers. A CRC, or Cyclic Redundancy Check, is a simple method for detecting the presence of simple bit-inversion errors in a transmitted block of data. In SD 4-bit mode, the input data is multiplexed over the four bus (DAT) lines and the 16-bit CRC is calculated independently for each of the four lines. In an all-software implementation, calculating the CRC under these conditions can be so complex that the computational overhead may mitigate the benets of the wider 4-bit bus. A 4-bit parallel CRC is trivial to implement in hardware, however, so custom ASIC or programmable-logic solutions are more likely to benet from the wider bus.

http://alumni.cs.ucr.edu/~amitra/sdcard/Additional/sdcard_appnote_foust.pdf

I believe you wouldn't have enough power to calculate crc32 in 4-bit mode. But without CRC it might be possible.

The "checkbusy" must send 0xFF to the SD card (that is not clear in your question). Is this SD or SDHC? The latter have some differences in initialization and operation sequences.

If we don't send dummy data after read ...

In SPI, you have to send data to be able to read from the client. Remember that the master always drives the clock line. The dummy data I send when reading SD cards is 0xFF, too.

The 512 zero bytes may be actual data from the device. Are you sure that you sent the address bytes in the correct order? The SD Simplified Specifications are available from the SD Association.