How to partition an SSD that fails with error 69888 - couldn't unmount disk?

I can't test it but I think that if you erase the MBR (with dd instead of fdisk) and the partition table record you should be able to re-format you ssd. Boot from cd or external hd and open terminal

this code erase the MBR:

# dd if=/dev/zero of=/dev/disk0 bs=446 count=1

this one erase the partition table also:

# dd if=/dev/zero of=/dev/disk0 bs=512 count=1

If you erase just the mbr you could use testdisk to attempt to recover data (in case you don't have erased the disk label).

I got the same "error 6988 couldn't unmount disk" message. It was due to unclosed references to the drive/partition - particularly I had a terminal window open and the PWD was on that volume. Close that window and close all Finder windows that has reference to that volume (or simply cd to a different directory outside of the mounted disk tree), redo the steps, it should work.